We've decided to get aggressive about social platforms here at PK. For a while, we've had a facebook group, and now we've started a Ning Public Knowledge group. Each offers something a little different. At facebook, you've got the large communities of people who are already invested in the platform. I'm hoping that they allow for group administrators to embed applications into group pages (until they do, we'll keep manually inputting our content). That's actually what I think the Ning group offers more of, is the ability to syndicate content.
Please let us know what kind of content we can provide to spur discussion and keep things interesting. But really, the social network is all about the interaction. So, please join us on Ning and facebook!