Mac has added .Mac Groups to the DotMac service. I’ve been a Dot Mac user for as long as the service has been available. It’s great…if you’re a Mac user. A wonderful addition to social software…if you’re a Mac user. The limitation is that you have to be a Mac user – a .Mac user in fact.
The book that I’m presently writing working on focuses on the power of social software to impact communities. Services like Meet Up and the much less expensive LiveJournal (part of SixApart) are cross platform and probably a better bet for most people. That is until Steve Jobs has us all assimilated.