Yes, this is about web analytics but statisticians (like me) do segmentation analyses too. For the month of May 2010, Gigya's Identity Study reports which sign-on's and ISPs were used for major traffic sites, stratified by category. Category types are more varied than just social networking, although Facebook and Twitter are included. I haven't checked the methodology. I did see that the sample sizes were sufficient, and that comparisons are provided in percentage terms and absolute values. There's the requisite infographic, of course.
Social Relations and the Internet
For more trend following, see Pew Research's 2010 update about the future of social relations online.