Though again atop the rankings of US social-networking sites, MySpace.com‘s audience grew just 4% in February from Feb. ’07, whereas second-place Facebook‘s more than doubled, growing 102% year over year, according to custom lists compiled by Nielsen Online.

LinkedIn, meanwhile, increased its unique audience a whopping 271% year over year, reaching 7.4 million in Feb., compared with 2.0 million a year earlier – and challenging fourth-place Windows Live Spaces.

MySpace’s unique audience totaled 55.4 million in February, more than double Facebook’s 20.0 million.

Top Blogger Sites

Google‘s Blogger remained atop the blog site rankings with 37.2 million unique visitors, up from 23.6 million in Feb. ’07 – or a growth of 58%.

WordPress was up an impressive 209% year over year, increasing its unique audience to 16.5 million.

Fifth-place TheHuffingtonPost.com’s growth was also impressive: Its unique audience grew from 1.1 million in Feb. ’07 to more than 3.7 million – or 241%.