Relative size & growth of Facebook vs Open Social networks (US only)
see my related post on Google Open Social vs Facebook Platform
note: in the graph above, the "OpenSocial" line represents the following networks: hi5, friendster, orkut, linkedin, plaxo, & ning sites; re: Bill Tancer's blog post on Hitwise.
don't believe it includes OpenSocial partners RockYou & Slide & others, who arguably are networks of their own at this point. however, much of the Slide/RockYou userbase has been initially collected from MySpace & Facebook.
finally, this analysis does not consider if/when Google enables Gmail / Gtalk / iGoogle users to opt into the Google social platform, or additionally if YouTube users were enabled as well. any of those scenarios might bump up the numbers for OpenSocial quite a bit.