• by Erik Sass , Staff Writer @eriksass1, August 21, 2012

Facebook had fewer unique U.S. visitors in June 2012 than it did in June 2011, according to comScore, marking the first time that the world’s dominant social network has experienced a year-over-year decline.

In June 2011 Facebook attracted 160.9 million unique visitors; the number declined by 1.1 million, or 0.7%, to 159.8 million unique visitors in June 2012. The peak number of users was achieved in January of this year, when Facebook attracted 168 million unique U.S. visitors, also per comScore.

The June year-over-year decline follows a sequential decline in March-May of this year, when Facebook’s unique visitors slid from 158.9 million in March to 158.7 million in April, and 158 million in May.

As far as the year-over-year drop in June, there are a couple caveats which should noted: first of all, comScore revised its methodology in July 2011, which may make direct comparisons difficult. Second, the comScore figures might not reflect the growing number of people who access Facebook via mobile devices -- which came to 78 million Americans in May, according to the “Mobile Metrix 2.0” report released separately by comScore earlier this year. That’s a lot of people using mobile devices to access Facebook, which could obviously change the year-over-year comparison, depending on the degree of overlap between mobile and desktop users.

These caveats notwithstanding, at the very least the comScore numbers make it clear that Facebook’s growth has slowed dramatically in the U.S. They also raise the possibility that interest in Facebook is waning, if only through the desktop channel -- which is a problem since it still represents the large majority of Facebook revenues.