My rule is to use Facebook three times a day for twenty minutes each. I'm not addicted to FB, but this helps me stay away from long chats I may get into with friends. I get shit done with this method and I rarely break this rule.
I'll stop my inflammatory rant because I'm going to assume you're a reasonably smart cookie because you're here. But really?
*for a 6 month personal experiment I deactivated my FB account and defriended, 2% of current friends per reactivation (17 times each under an hour) and could argue that I still have a top 5% social life.