Nielsen//NetRatings reports the top social network portals for September 2007:

Top social networking sites in September 2007
  Audience, 000 Growth
Site September 2006 September 2007 YTY
Myspace.com 47,189 58,581 24%
Facebook 7,765 18,090 133%
Classmates Online 13,291 13,313 0%
Windows Live Spaces 7,782 10,275 32%
AOL Hometown 9,997 7,685 -23%
Reunion.com 4,712 4,845 3%
LinkedIn 1,762 4,075 131%
AOL People Connection 6,096 4,017 -34%
Club Penguin 1,333 3,769 183%
Buzznet.com 1,061 3,737 252%

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(Via IT Facts @ ZD Net)