I have a coworker who deactivated, then I made him reactivate 3 years later (because we were working on a project that used FB for login, so he had to have an account to work on it). It came back like nothing had happened. He spent that afternoon deleting all of his friends & adjusting his privacy settings to make himself as invisible as possible.

Interesting side effect: while unfriending everyone (except for me, since he had to have at least one friend to test the project we were working on), he unfriended his girlfriend. This ended their relationship status (which had come back into being when he reactivated). This resulted in a "______ is no longer in a relationship" story on her wall. This resulted in friends & family calling her and asking if she was ok and offering to listen if she needed to talk. It took her a few hours to figure out that they all thought she & he had broken up. She was also not very happy when she found out he had unfriended her but not me.