I'd say it's because of inertia. The same network effect that sucks you in to FB, because most people around you expect to find you there, raises the barrier for leaving.
However, social trends are largely about perception. Imagine this: a popular musician says publicly, "Facebook is evil and lame, I'm leaving." Lots of teens say, "hey, yeah, me too. I'm a nonconformist. Facebook is lame. I'm leaving." Suddenly leaving Facebook is a cool thing to do, and the network effect is working against them.
Eventually it's like AOL or MySpace: having an active account is definitely uncool.
However, social trends are largely about perception. Imagine this: a popular musician says publicly, "Facebook is evil and lame, I'm leaving." Lots of teens say, "hey, yeah, me too. I'm a nonconformist. Facebook is lame. I'm leaving." Suddenly leaving Facebook is a cool thing to do, and the network effect is working against them.
Eventually it's like AOL or MySpace: having an active account is definitely uncool.