It's not that leaving would restrict your social life so much as that FB enables a much looser social network to coordinate. I'd stay in contact with my friends

It's not that leaving would restrict your social life so much as that FB enables a much looser social network to coordinate. I'd stay in contact with my friends, but I'd see a lot less of what my friends-of-friends and acquaintances are doing. Those more distant social interactions are less valuable to me, sure, but there is some value in them and there are a lot more of them.