Here is a great list of powerful metrics to use on social media. I've culled these from Gary Angel's Semphonic presentations which are available here.
Community Metrics (like Facebook or Google+)
•Incremental Reach: New visitors to the Community from Non-Branded Natural Search
•Effective Reach: Direct Community Traffic + Non-Competitive Search Traffic Visits/ All Visits
•Community Independence: Visits sourced on a main site that drive to a community / Total community visits
•Community Integration: Visits sourced on the community that drive to a main site.
•Direct Marketing Effectiveness: Visits that include community and a main site measure of success/ all community visits
•Halo Marketing Effectiveness: Visitors that include community and a main site measure of success / all visitors
•Issue Rate: Problems by Feature of Application / Total Problems
•Feature Requests: Request count by Feature