I bet most users wouldn't leave facebook for $12. Interestingly I doubt most would pay $12 for the service, either.

I am not on Facebook at all, even if it is free because I just haven't seen the need for it. I might develop some apps for it in the future, but that's about it. I'm not on Twitter, Friendfeed or Myspace either...

For me, the value of Facebook is 0

over their lifetimes on facebook? i'd bet that facebook could easily get a substantial fraction of its users to pay that much every two years or so; whether they want to go that route is up for debate.