Hmm. That's a great way of looking at things!
Since I don't have that many IRL friends and a small family that keeps dwindling and school friends I'd really rather not make contact with, plus I don't use my real name online anyway, Facebook doesn't do a lot for me. They *do* have some good private groups I joined (*SMASHbooks, ukulele interests, tarot).
Twitter is almost more a news service for me. I follow a LOT of weather folks. My feed blows up every time we have severe weather. :-p I like the immediacy of Twitter.
So I guess I need to think out what I want from the various places I hang out. I really like the way you think about this. :-)