Powermingle:

"A great network is a valuable asset: It secures a steady flow of opportunities - it's the typical way of finding a new job - and it's a personally stimulating way to get to know a lot of nice, interesting people. It is therefore hardly surprising that building a good network is important to us all."

This networking site uses .Net Passport to manage sign-on (here's a directory with more sites doing so). Powermingle takes a different approach than Rize: they take a long time explaining how great they are, instead of letting you "taste" and use the site as fast as possible. But Powermingle might be more of a competitor to Meetup than Rize, as an online complement to offline events. And since the web is a fun place, Powermingle's CEO is giving Rize a try. (Update: this account has since been cancelled).