As recently as 2010, growth in US Facebook usage was well into the double digits, at 38.6%, eMarketer estimates. But with 116.8 million US internet users already logging on to the site at least once monthly that year, growth rates were bound to plateau.By 2011 Facebook user growth rose a comparatively small 13.4%, and this year will be the first when growth rates drop to the single digits. Rates of change in the US will continue to decline throughout eMarketer’s forecast period.

via Facebook’s US User Growth Slows but Twitter Sees Double-Digit Gains – eMarketer.