Among the 1,542,612 English-language pages of Wikipedia is a list of "notable social networking sites." Usefully, this list includes the most recent published data for the number of registered users in each network.
Just for fun, I sorted the list by size. Here, in descending order, are the top 20 social networking sites based on this data:
The full list of 76 sites is here. I have no idea how accurate the data is -- most of it is self-reported -- but if you're looking to anatomize social networking sites to come up with ideas for your own community or network, this list isn't a bad place to start.