Ah, the perils of keeping up with the zeitgeist. No doubt when Julia Angwin began writing “Stealing MySpace” she was confident that MySpace was indeed “the most popular Website in America,” as the subhead claims.

Alas, that no longer seems to be the case. In recent months, Facebook traffic has surged to at least near parity Stateside, with Twitter coming on strong among the media and political classes. MySpace now seems so old hat that a reference in “He’s Just Not That Into You” sounded hopelessly dated (and therefore uncool) rather than hip and knowing.

Frankly, I questioned the point of Angwin’s book at this juncture. When I finally picked up the galleys, I was surprised by how doggedly the Wall Street Journal scribe clung to her premise. Yes, Facebook is growing, Angwin wrote more than once, before dismissing it as small potatoes compared to the all mighty and popular MySpace.

Except that’s not the case anymore. Last year Facebook passed MySpace worldwide and some of the most recent metrics suggest it has done the same in America. Its growing legion of middle-aged converts recently gained the attention of Time.

What I don’t get is why Angwin didn’t couch her claims a little better — especially as it became clear that Facebook was making such rapid gains last year. But I guess that would dilute the significance of her undertaking.

It’s too bad she didn’t do more to address those issues, for there is much of interest in the book. There are eye-opening stories about founders’ seedy dealings in spam and spyware, and plenty of juicy corporate intrigue following the News Corp. purchase.

Books about fast-moving developments are tricky, as I’ve noted before. But in order for these books to be as relevant as possible, writers and their editors have to be able and willing to adjust on the fly, just as a series of authors did when the economy plummeted last fall.

Here’s hoping that HarperCollins has greater success reacting to Internet time, if that phrase itself isn’t too hopelessly passe, with its new It Books imprint than Random House did with Angwin’s tome and “Stuff White People Like,” a lackluster spinoff from a popular blog. First up, per the NYT: “Twitter Wit,” a collection of posts, all no longer than 140 characters long.

Question is: Will we have had our fill of Tweets by then? By the time various movies about Facebook hit the bigscreen we will no doubt have moved on to something else.