It's about three days that I've felt in a large bunch of these networks and my huge interest to join them and test them.
I tested about 5 of them in just one day. The great things here are the APIs which connect all of them together, even your Yahoo!, Google, AOL and etc. This is why I inserted '++' in the Title.
You know, this is the Cloud. A social cloud which everybody can use in his own method, Social or Professional.