What, out of curiosity, would prompt them to leave a service that all their friends use? There needs to be something so useful that I'll join a new site and leave behind all my old friends. I don't see that happening with Facebook.
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.