Instant Web Hit

Sam: Look, see the count? Only 27 people have clicked on it.
Carly: Thank God. [pause] Sam?
Sam: Yep.

Carly: THAT'S 27 THOUSAND!

—iCarly pilot

We know that It's a Small Net After All but even though Everything Is Online already, when Bob uploads a video or a Web page, especially if this happened by accident or the contents are particularly embarrassing, expect it to get more hits on its first day than a successful non-profit site can hope to get in weeks.

For videos, this is often the result of an Instant Humiliation - Just Add YouTube ploy.

This is occasional Truth in Television when a new website catches the eye of a large news site, or a video becomes a viral internet meme. Even when it does occur, it will usually take at least a week to even be noticed by enough people to become popular. It is possible, but obviously both rarer and slower in real life.

Examples of Instant Web Hit include:

Anime & Manga

Not a video, but the same type of thing: In the second Negima anime, Nodoka becomes the Internet's #1 idol after someone posts some pictures of her online. Chisame is not happy about this.
Doing this is the focus of one of the competitions in the Kujibiki Unbalance OVA. The opposing team of Otaku post pictures of anime girls and get large numbers of hits, while the main characters make feeble attempts at websites about trains and such and get few hits - until they decide to post video of the girl characters in skimpy outfits and win.
In Death Note, the cult following for Kira starts on the Internet - in fact, it's the Internet followers who give him his name.

Film

At the climax of the Marmaduke movie, the title dog character gets rescued by his owner Phil. Nearby kids had filmed the rescue and put it on YouTube.
The "Granny Panties" video of Miri in Zack and Miri Make a Porno got 300.000 hits in the first few hours.
In The Social Network, Mark Zuckerberg's site "Facemash", created while drunk, gets enough hits within a matter of hours to shut down the Harvard University servers. Truth in Television, as this actually happened, and is likely the most accurately-depicted event in the movie.
In Bajrangi Bhaijaan, Pakistani reporter Chand Nawab, frustrated because the network where he works doesn't want to transmit his story about an Indian man (the titular Bajrangi) that illegally entered Pakistan to bring a stranded Pakistani mute girl back to her parents on accounts of not being "interesting" enough, independently uploads it to YouTube with the addenda that the man was arrested just after successfully finishing his mission. It becomes instantly viral in both India and Pakistan, so much that the popular support it generates forced the Pakistani authorities to release and deport Bajrangi instead of sending him to trial.

Literature

In Michael Crichton's Next, one character creates a fictional Web page to fool her daughter. Within minutes, this page is the top Google hit.
In Son of the Mob, Vince creates a boring, useless webpage about cats for a school project. The page ends up getting thousands of hits, which confuses everyone. Later he finds out his brother was using the page to run a gambling ring.

Live Action TV

In the iCarly pilot, the funny video that started it all has an amazing 27,000 hits on the first evening it has been online.
The The Big Bang Theory has used this trope a few times:
- Leonard makes a Web page for Penny to sell her small crafts project. While the others are still discussing the merit of the site, an order of 1000 units comes in.
- When Leonard and Sheldon have a fight during a physics lecture, the video Howard shot becomes a YouTube featured video the same evening.
- Sheldon gets drunk and makes a fool of himself at an awards presentation; this becomes a huge hit on YouTube by the next morning under the title "Physicist has a meltdown".
On Lizzie McGuire, Matt's webcast from his basement gets a ridiculous number of hits when his father accidentally turns it into a slapstick routine.
Averted in Glee: When Sue threatens to upload an embarrassing video of the principal, he tells her that he uploaded it himself a week earlier, and it only got two hits.
- Played straight - and a little karmically - in a later episode, when an embarrassing video of Sue dancing to "Let's Get Physical" is posted on YouTube. It soon goes viral and within a matter of days becomes so popular Olivia Newton-John offers her a role in a remake of the music video.
- Played straight in YouTube videos "The Kissed That Missed" humiliation at Nationals (which goes viral worldwide) and "Mercedes Inferno" performance of Disco Inferno (several hundred hits is still pretty significant in a timeframe of less than a day).
Used in an episode of Leverage to convince the mark that he can make it big by hiring one of the team. Of course, this is a con - they set up a script to automatically view the video from many different IP addresses.
On In Plain Sight a teenager in witness protection performs in the school choir and one of the parents makes a video of it and posts it on the Internet. The video goes viral because the girl was an up and coming rockstar before witnessing a murder and she still has many loyal fans wondering what happened to her. Mary has to relocate the girl before the gangsters who want her dead come looking.

Video Games

Consciously averted in the .hack series, where it is noted at least once that the full Epitaph of Twilight was posted briefly online, but was taken down before many people even knew about it.

Webcomics

This Penny Arcade strip.
This Ph D strip.
This Xkcd strip.
This trope causes Rudy's problems in This Kevin and Kell episode.
This strip of El Goonish Shive has multiple You Tube videos get several thousand viewings in less time than it takes the news crew to arrive by helicopter.
Phillipe's disastrous Public Service Announcement goes viral in this Achewood strip.
Dissonance: Gen uploads some footage of Pandora to YouTube, and gets over 4.7 million hits overnight.
Unwinder's Tall Comics: Unwinder has a friend film him while he crashes a sled, and he intends to put the video up on youtube. An Imagine Spot ensues, where the video gets 14 million hits, and Unwinder himself becomes an international punchline who no one wants to hire.
As Jamie finds out in Girls with Slingshots, her drunken "I just want somebody to ravage my body" outburst becomes a YouTube hit and a meme overnight.
In Leftover Soup, Ellen is Answering Machine Bitch.

Western Animation

In the Phineas and Ferb episode "Lights, Candace, Action", Phineas and Ferb's video of Candace gets a million hits almost immediately - slightly justified because the two were apparently already popular Web directors.
- Another Phineas and Ferb example is in episode "Tip of the Day": When Dr. Doofenshmirtz accidently showed his embarrassing video to the internet and wanted to erase their minds. It was so famous that he encountered judgemental paparazzi just when he went outside his lab (and that was only to his kitchen!)
In the Mickey Mouse Works short computer.don, Daisy sets up a Web site with an Embarrassing Old Photo of Donald Duck, which gets a million hits in five seconds.
In an episode of The Simpsons, Bart puts his Angry Dad cartoon on the Web, and it becomes the #1 non-pornographic site on the entire Web (making it 17 billionth overall).
Foster's Home for Imaginary Friends had the embarrassing "Funny Bunny" video.
In The Penguins of Madagascar, Private's accidental fall when stepping on a roller skate (It Makes Sense in Context) turns him into an internet celebrity overnight, until Disaster Dominoes turn the camera on to the female zookeeper doing a silly dance while thinking she was alone.

Real Life

A well-known example: Justin Bieber became a teen idol after a manager found one of his videos on YouTube, and it involved getting millions of views as well:

After posting dozens of homemade videos on YouTube in 2007, where the multi-talented Bieber put his impeccable spin on songs from artists like Usher, Ne-Yo and Stevie Wonder, Justin racked up over 10,000,000 views purely from word of mouth.

It's usually commented that any video uploaded by Raocow, or having Raocow as one of its tags gets 3,000 to 4,000 views by the end of the day. This includes demos, silly cuts, audiosurf songs, or whatever.
Happened to Red vs. Blue, believe it or not. Apparently, Rooster Teeth had more than 2 million downloads within a day of posting the first episode. They got popular enough that it took Bungie only a week to notice them. Fortunately, Bungie loved the series (it was essentially free publicity for Halo after all), and did not shut them down.
Although some criticism and controversy surrounded it, the video Kony 2012 by Invisible Children certainly succeeded at raising awareness of the Ugandan war criminal Joseph Kony (notorious for his use of Child Soldiers). The 30-minute documentary racked up over 100 million views in just six days. According to those who measure such things, it's the fastest-spreading viral video in the history of the internet—all the more impressive when you consider that it's a long, serious piece that doesn't involve kittens.