Facebook's growth rate is slowing

Facebook's growth appears to be slowing, particularly in the U.S., according to a report in the Wall Street Journal.

Unique U.S. visitors to the wildly popular social media site rose 5 percent in April to 158 million, according to data attributed to comScore. ComScore confirmed the data.

That's the slowest growth rate since comScore started tracking data in 2008.

Users spent more than six hours a month on the site in April, up 16 percent from the prior year. Still, that's a slower growth rate than the 23 percent increase in 2011, according to comScore data cited in the report.

Facebook went public on May 18 in a widely anticipated market debut. But the stock price has slid since then. Facebook's stock was up about 2 percent to $27.65 on Monday afternoon.

Investors are concerned about Facebook's ability to keep increasing revenue and make money from its growing mobile audience. Meanwhile, many analysts hold positive long-term ratings on the stock.

The company has been rolling out new offerings for mobile users, including apps for taking photos, messaging and for managing business brand pages.

It is also unveiling an app center to allow users to find Facebook-related games and other applications more easily.

Facebook Inc., based in Menlo Park, California, has nearly a billion users worldwide.

On Tuesday, comScore will publish a report measuring the effectiveness of advertising on Facebook. The report, previewed on a comScore blog post last week, also looks at other things brands do to promote themselves on Facebook beyond paid advertising. Such "earned media" can include Facebook users sharing a post about the brand.