Social Networking for Project Managers, sounds something great.
http://www.projectmanagers.net/ this is a site for SNS among project managers, i like the concept.

As a Project Manager you can get register and share your views about how you manage you projects, or even you can post your concerns among other project managers and find a solution to your problem. It a good platform to learn about new Project Management Methodology. If you are working on a waterfall model and think what is Agile Scrum then there are some good resource on the site which provides you some insight to that.

Overall its fun to be a part of this community

Here are some links that might be helpful to you:

What is this thing called
A Qquick Primer on Social Media
Six Common Mistakes Project