Year-over-year, Facebook's usership grew by 102% to just over 20 million unique visitors last month.
Also of note, business networking site LinkedIn grew an impressive 271% from about 2 million unique visitors in Feb. 2007 to 7.4 million unique visitors in Feb. 2008.
Club Penguin, Disney's virtual network for kids, grew 46% year-over-year, according to Nielsen--to about 4.7 million unique visitors in February.
Viewership of Windows Live Spaces, meanwhile, was down 15% to about 7.9 million unique visitors in February.
--Gavin O'Malley