Great list comparing tools for social learning (Twitter, Facebook, Ning, and Elgg). With Ning's new pricing plans, looks like for most non-personal uses, Elgg comes out ahead, however it either has to be downloaded and installed (does that mean your machine would need to be on all the time for folks to use your site?) or you can pay for hosted service.
Google Wave is a tool that allows asynchronous communication (similar to email or discussion boards), semi-synchronous communications (similar to Twitter or FriendFeed) and synchronous communications (similar to instant messaging) all wrapped up with wiki-like capabilities for collaboration.