I can see why these sites are so popular - they are great places to invent (or re-invent) yourself online in relative safety, but as with so much of this social networking technology you still have to wade through the usual legions of adolescent diaries and lame groups where they all argue about the virtues of evlish versus klingon, or whether Bladerunner or Star Wars was the greatest SF movie in the universe to get to the bits worth looking at. I'm still not sure I want so much personal info just out there for the world to see. Back when we all wrote our diaries the old-fashioned way they often had keys, or you hid them where your Mum and your sister couldn't get at them. It was called privacy, and back then, I valued it.
I've had a look at the three sites, I've created an account and a basic profile (Facebook) - but do I feel moved to keep using the site? Proably not, for now anyway. I'd rather talk to actual friends in a real cafe while drinking coffee I can smell and taste - the virtual world is fun, and occasionally useful (see last post) - but I still prefer the offline world. It has better bandwidth.