American Gods/WMG

Shadow isn't the
That was a full god, and Shadow is only a demigod. There is a running theme of one thing being two and two things being one in the American Gods continuity, so Shadow could easily be a demigod who was ironically (the intentional version of irony) given the same name as his Deader Than Dead half-brother, and accidentally stepped into his half-brother's place in things-- picking up a few extra tricks along the way.
 * Even if he is the way gods work in this universe there would still be a "real"  in Scandinavia: Shadow is the American   as Wednesday is the American Odin.
 * My point was, he's not the American one. It's fairly obvious that he's not the original Scandinavian one, the same way no one (aside from, possibly, native American gods, and probably the New Gods) is the original _______ one.

The book and the twitter are an attempt to restore faith in the gods.
After the battle the new and old gods were too weak to exist without some additional belief, so they first got Neil Gaiman to write the book and later the god of social networking started the bookclub on twitter to keep the interest in the book high.

As a corrolary, this book was made by the New Gods to ensure their survival
Possibly the reason the Old Gods continue to survive no matter what, while the New Gods die when the thing they repersent go out of style, is because the stories of the Old Gods are still being told. Smithing may not be as mainstream as it used to be, but as long as people continue to pass down stories he is in (such as the origin of the Greek gods), Vulcan will still survive. Railroads didn't have that advantage, so he died when not enough people took the train. Cars will not have that problem, so long as people read this book.

The two guesses above are both true; additionally, the whole idea was Wednesday's Plan B
Because logic dictates that a Bastard as Magnificient as Wednesday can't go without contingency plan. He had made arrangements so that when  and his "war" went downhill, a record of the whole thing is delivered to Mr. Gaiman. The resulting faith brought forth by the finished novel was enough to nourish both the Old and New Gods and resurrect their dead (including ). Wednesday could then claim this is what he was aiming for all along, honest.

Mr Wednesday is Just Hiding.
Although Shadow stopped the battle proper, gods still died in the opening skirmish, including between Shadow finding out it’s a con and talking to the dis-embodied Wednesday and stopping the battle: possibly enough to fuel Wednesday's resurrection. Given his plan was scuppered by Shadow before he could gain full power, he decided to lie low and play dead.
 * Well, Monarch of the Glen suggests he still exists on some level at least, unless Shadow was just dreaming. Which, given as this Neil Gaiman we're talking about, is unlikely.
 * "It is never just a dream."

Bast took over The Internet's position after the latter's death.
It explains the popularity of LOLcats.

Loki is/was J. Edgar Hoover.
He infiltrated the FBI during its earliest stages and cultivated it into something iconic. While in a position of power, he also engaged in randomness and rumor mongering so that people would still talk about the scandalous little details to this day - up to and including the belief that Hoover had nude photos of Elanor Roosevelt "just in case".

At the same time, he actively worked against the reputation of that individual organization in preparation for his retirement, making sure to discredit it and any of its successors. Which would mean that he was the one who first told the CIA joke. By doing this, he was able to build up a suitable MIB mystique in American consciousness that it manifested the New-Gods-aligned organization that we all know & hate. Using his experience as Hoover, Loki took control of the organization and was able to reap the benefits of having an army of disposable little gods that are easily dealt with when faced with a Tomato in the Mirror moment.

This is why Mister Town & co follow his orders. They know that he was Hoover before becoming Mister World, and that's explanation enough for them. The higher-ups always know better & are more scarily-competent. Even when the higher-ups are in jail.

Lakeside is Lake Wobegon
Both are towns (beside a lake) in the Midwest that have thrived despite the depopulation of the rural Midwest. There are several other similarities, such as the genial and occasionally eccentric inhabitants.

The god in Las Vegas was Binbougami, the Japanese spirit of poverty
Explains why he gets empowered by the loss of wealth and why he would request Soma as his price.
 * For most theories concerning the unknown god see this page. It is impossible to tell and Gaiman has said he will not say anything on the topic.

Low-key has some connection to Verbal Kint of The Usual Suspects
Let's see- Scheming character with multiple identities, one of which is the persona of a two-bit con artist. Both arrange their arrest so they can meet up with people in prison. The two admittedly look pretty different, but some amount of disguise going on seems pretty plausible.

American Gods and Eight Days of Luke take place in the same continuity.
Gaiman has said he was inspired by Diana Wynne Jones, and Luke and Wedding have very similar personalities to Low-Key and Wednesday, just toned down a little for a children's book. Also, at the end of Eight Days of Luke, Wedding mentions battling other faiths for belief. And the Valhalla hidden inside a funfair is similar to various settings in American Gods.

Thor is alive because they're the British versions, not the American ones, and are doing a little better (or were back in the seventies.)