This data is a little messy (forgot the link) but very useful
Market Share of US Internet Visits to Top 20 Social Networking Sites February 2007
Rank Name Domain Market Share
1 MySpace www.myspace.com 80.74%
2 Facebook www.facebook.com 10.32%
3 Bebo www.bebo.com 1.18%
4 BlackPlanet.com www.blackplanet.com 0.88%
5 Xanga www.xanga.com 0.87%
6 iMeem www.imeem.com 0.73%
7 Yahoo! 360 360.yahoo.com 0.72%
8 Classmates www.classmates.com 0.72%
9 hi5 www.hi5.com 0.69%
10 Tagged www.tagged.com 0.67%
11 LiveJournal www.livejournal.com 0.49%
12 Gaiaonline.com www.gaiaonline.com 0.48%
13 Friendster www.friendster.com 0.34%
14 Orkut www.orkut.com 0.26%
15 Windows Live Spaces spaces.live.com 0.18%
16 HoverSpot www.hoverspot.com 0.18%
17 Buzznet www.buzznet.com 0.18%
18 Sconex www.sconex.com 0.14%
19 MiGente.com www.migente.com 0.11%
20 myYearbook www.myyearbook.com 0.11%
Note - data is based on custom category of 20 of the leading social networking websites ranked by market share of visits, which is the percentage of traffic to the site, based on Hitwise sample of 10 million US Internet users. The percentages represent the market share of visits among the websites within the custom category.
