Facebook’s membership is nearly four times that of Twitter.

The number of Facebook members in the U.S. is about 152 million; Twitter has roughly 38 million users.

However, a new report from eMarketer reveals that Twitter’s growth is exploding in the U.S., as Facebook’s slows to single digits for the first time. Overall, the growth in membership for both sites is expected to decline over the coming years in the U.S.

According to eMarketer, Facebook membership grew by 13.4 percent in 2011 (a 25 percentage point drop from the previous year) compared with Twitter’s 31.9 percent growth (an increase of roughly 8 points from 2010). This year, Facebook users are expected to grow by 6.6 percent; Twitter’s membership will likely expand by 20.7 percent.



The report blamed Facebook’s tapering off on the huge number of members it already has in the U.S. and cited Twitter double-digit growth on its size.

From eMarketer:

“Twitter’s size, which is fairly small, is one factor that makes such growth rates possible. Facebook already reached an enormous audience of nearly 133 million U.S. Internet users at the end of 2011, a figure that will surpass 150 million by 2014. Twitter, in comparison, had a U.S. user base of less than 24 million at the end of last year. Still, between 2010 and 2014, eMarketer predicts, Twitter will about double its U.S. user base, reaching 37.6 million microbloggers by the period's end.”

Last year, Facebook questioned a report indicating the social network had lost 7.5 million users in the U.S. and Canada. That report, like others, stressed that Facebook continues to grow abroad.

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