Even though Facebook is a social networking juggernaut, it may be reaching the limits of its growth in the United States. Reuters notes that comScore’s latest numbers show that Facebook notched 158.01 million unique U.S. visitors this past month, down from 158.69 million in April and 158.93 million in March. While this obviously doesn’t mean that Facebook is about to come crashing back to Earth anytime soon, it does suggest that the social networking site may have reached the peak of its appeal for now. Not that that’s a bad thing, since comScore also found last month that Facebook users are spending ever-more time on the site, logging an average of 380.8 minutes in May, up from an average of 378.9 minutes in April. Not bad for a site deemed “useless and boring” by a third of its users.