Last month, Facebook attracted 158.01 million unique visitors in the United States, edging lower from 158.69 million in April and 158.93 million in March, comScore said.

Keeping users coming back -- or combating fatigue -- is crucial for all social media services, analysts say. Facebook is consistently trotting our new features, including the "Timeline" interface rolled out this year, and more are expected with the deal to acquire popular photo-sharing app Instagram.

ComScore has changed how it counts users, making year-ago comparisons harder. Under its old methods, comScore previously said Facebook had 157.22 million visitors in May 2011, which would make Wednesday's data a year-on-year increase of just 0.5 percent.

The changes comScore made generally reduce user numbers, so in an apples-to-apples comparison, user growth would look slightly bigger, a comsScore spokesman said.

Users spent an average of 380.8 minutes, or more than six hours, on the site in May this year, up slightly from 378.9 minutes in April.

In April last year, as measured under comScore's old techniques, users spent 374.9 minutes on the site.

Facebook was heavily criticized for the handling of its initial public offering in May, and critics have also questioned the efficacy of its ads.

One oft-cited reason for buying Facebook stock was the company's rapid growth.

The company's shares debuted at $38, but dipped well into the $20s before recovering some ground in recent weeks. On Wednesday shares closed at $31.60, down about 1 percent.

About two out of five people polled by Reuters and Ipsos Public Affairs said they used Facebook every day. Nearly half of the Facebook users polled spent about the same amount of time on the social network as six months ago.

(Reporting By Sarah McBride; Editing by Bernard Orr and David Gregorio)

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