This year, eMarketer forecasts, 132.5 million US web users will use the site. That increase of 13.4% in the number of users means Facebook will reach almost nine in 10 social network users and 57.1% of internet users. By 2013, 62% of web users and almost half (47.6%) of the overall US population will be on Facebook.
Facebook's broad reach means that its once-dramatic growth rates are over; eMarketer forecasts single-digit growth in users after this year. On Twitter, growth rates will be higher, but relatively few online Americans use the microblogging service.
By the end of 2010, 16.4 million US adults, or 9% of the adult internet population, used Twitter. Growth will surpass 26% this year as Twitter reaches 11% of internet users and 16.5% of US adult social network users. By 2013, nearly 28 million Americans will be tweeting.