Facebook Inc. went public last month on its prospects for growth. But in some key areas, the social network's growth rates are already maturing.

In particular, Facebook's user growth rate in the U.S. is slowing sharply. In April, U.S. unique visitors to the website increased to 158 million, up just 5% from a year earlier, according to research firm comScore Inc.

That was Facebook's lowest U.S. user growth rate since comScore began tracking the data in 2008 and was down from 24% growth in April 2011 and 89% in April 2010, comScore said.