I think of Facebook as a market on a street. Everyone is there. Google+ is a shop on the market street where you can get away from commoners, and Twitter is a small, indoor market on the street which is smaller but more controlled than the street outside. At least, that's what it looks like in my mind :P I much prefer Google+ for discussions with randomers about mundane things, while Facebook is literally for friends. Twitter is like a place for news and interesting updates on things that interest you.
To me, Google+ is better, but part of me likes Facebook purely because more of my friends use it.
