It certainly is more difficult to leave Facebook if many of your friends are there to stay. Actually, the social costs of leaving or not joining Facebook can be quite high. This may explain why people hesitate even though they would like to leave.
'Yes and I want to stay' of course does not mean that you don't know a single reason to leave. It means exactly what you are saying: That the pro-arguments outweigh the counter-arguments.
Think of it as lock-in.