Facebook currently has a problem that plagues only the biggest online services out there, but a serious one nevertheless: it's getting too big. Its growth hasn't been spectacular in the last couple of months like it once was, but one has to wonder if it's simply nearing the natural limit for that type of service. Once you've conquered a huge portion of domestic (US) users, and international users, where do you turn to?
Existing users. Facebook's algorithms, while still not perfectly tuned, recognize that some users aren't using Facebook regularly, and entice their friends to reconnect with them; write on their wall, send them photos, and the like. It's the perfect campaign. If a company sends you an email that says "hey, you've been inactive, but why not give our service another go," you'll probably disregard it. But if a friend posts a photo of you or some of your friends, and the only place you can get it is Facebook, you're far more likely to log in - and get hooked - once again.
I'm an active Facebook user, and this latest strategy is also working well on my end. I see suggestions for friends I already have; I'm interested in why Facebook's suggesting them; I go check out why they haven't been active; in the end, I really did reconnect with some of them.