Hmm, like the quote says, remember Friendster? Anyway, the same things that prompt all of us to drift apart from old friends, I should think. New friends and social circles, different lifestyles, different jobs, different priorities, different interests, etc. The same things happen online. I'm sure many people have irc channels/forums/games/sites they used to visit daily and members from that site they used to confide their deepest secrets to. I know I have. Now they're all no longer a part of my life.
I used to visit Facebook daily too, but I've mostly stopped visiting it, except when someone uses it to send me event invitations or someone tells me to go view pictures there. For everything else, there's IM. The thing is, if someone is really a friend of mine, then I don't need Facebook to stay in touch with them. If not, then I don't need to stay in touch with them, so sites like Facebook really had no intrinsic draw for me besides novelty or perhaps a business opportunity.
Now my main guilty pleasure is HN, and none of my friends are here. Then again, I am a bit of a loner, so this could just reflect my own atypical usage patterns and you could well be right in that most people will use Facebook for the rest of their lives. I doubt it though. I don't think Facebook addresses a need as fundamental as Google does.