I think a better metric would based on actual user behaviour rather than a simple count. For example, the number of posts or comments across the network, or the number of friend requests sent (new connections made). I still use Facebook quite a bit, but I've definitely become more of a consumer than a participant - I don't think my own status has been updated in months. It's basically become YouTube/Reddit, but with content based on people I actually know rather than what's popular.