MySpace is still the largest social network site in the U.S. But, if Facebook's growth trajectory continues into 2009, it will surpass its rival domestically, per eMarketer.
Facebook's internal metrics show significant U.S. growth. In December 2007, the site had 21 million U.S. active users (members who had visited Facebook in the past 30 days).
In December 2008, the number of active users reached 43 million, according to data provided to eMarketer by Facebook.
Facebook is not the only fast-growing social network in 2008. LinkedIn grew 112 percent between November 2007 and November 2008, while hi5 was up 88 percent, according to comScore. Both sites, however, had far fewer users than Facebook or MySpace.