According to Nielsen Online, via the Grauniad, Facebook saw its first dip in UK usage - around 5 percent - between December and January. That doesn't stop it being the top social networking site in this country, though. It's worth noting that MySpace also saw a comparable drop, while Bebo is also down.
Some people have jumped on this story as a marker of the decline of Facebook. Personally I reckon it's completely normal for a site like that to spike then stabilise as it finds its long-term user base and loses those who do approach it more faddishly. I certainly know many people who are losing interest - but then again I know lots of people who are using it more and more to organise their social calendar etc. Either way, Mark Zuckerberg & Co. haven't yet found a way to make money off the thing, so even a slight decline in usage will be bad news for them.
If Facebook is genuinely starting on a faddish decline, then I can't see what the next fad - the next big thing in social networking - is going to be yet. I certainly doubt that social networking itself is going to go away anytime soon. Furthermore, I suspect that Facebook has become entrenched enough for the majority of its userbase to need a damn good reason to go elsewhere - as it stands, that would be needless and counterproductive fragmentation.
On the subject of Zuckerberg's Folly, a US company called WorkLight has brought its Facebook overlay, WorkBook, over to the UK now. This apparently makes it secure and enterprise-friendly, so if your organisation is currently blocking FB over security concerns, it might be worth a gander.