Most engaging social networks

I’ve started looking at what are the most engaging social networks for US users. Comscore lists 174 sites in its social networking category. I ranked the sites by two key measures of engagement:    

1. pageviews per user per month
2. visits per user per month

The most engaging social networks by PVs per visitor (I cut off the list for sites with less than 100 PV/User/month: Re-ranking by visits/month (and cutting off at 4 visits per month or once a week) gives the following list:

Re-ranking by visits/month (and cutting off at 4 visits per month or once a week) gives the following list: