What has happened to all the new social networks?

Where have all the social networks gone? Of course, this is exactly the right time to be asking this question. Haven't I noticed that Facebook is now claiming 500 million users, in the manner of Doctor Evil in Mike Myers's Austin Powers movies? Haven't I noticed that Twitter is getting its very own data centre, all the better to spread "unimportant trivia" (� all tabloid papers) such as the