FB gives you a great way to stay peripherally involved in people's lives. It allows you to have numerous but low-cost touch points such that when you next have a chance to properly catch-up you already know what the major events have been.

It's the complete opposite of email where I think "I'll wait until I have something proper to say" and then never send anything. On FB, it's totally normal to leave one-line comments. Having said this, I barely use FB anymore (due to privacy concerns).

Edit: I should also add that I've never used email in the social way that people seem to refer to. I really don't understand how that would be possible. FB allowed new types of social exchanges that didn't exist before (for me, at least).