I routinely post anything I want to be publicly visible to Twitter. It's then forwarded to Facebook, LinkedIn and (soon) G+. If I want something visible only to my friends, I post it to FB (I keep the friend list there tidy) - hopefully I'll be able to forward those to my "Friends" and "Family" circles on G+.
I don't want to maintain yet another stream whenever there's a new social network thing going on. OTOH, I don't post my "stream of consciousness" to Tw, I try to use it as a broadcast medium. I am annoyed by "today I had X for lunch" type posts, be they on Tw, FB or G+.