Ala LinkedIn and other popular services that encourage users to become more engaged and active, Facebook appears to have rolled out a new "activity" measurement for users. While other sites also help users advance through new features and relationships, Facebook's measurement is unique because it is public. For instance, the below unit appears in my suggested panel and lets me know how "active" a particular friend is. It then encourages me (very interesting!) to "write on her wall" or do other activities to make her more active.
It's a brilliant idea because Facebook, better than anyone, knows that social encouragement and activity are what drive long-term user engagement and usage.