A recent report from comScore showed that Facebook had reached its all-time high in audience size in the US in May, in contrast to other reports which said the social networking site lost a significant number of users in the month.

Facebook gained 3.2 million visitors in the US, taking its audience size to 157.2 million in May, said comScore.

Inside Facebook, a research firm, recently reported that Facebook users dropped by nearly 6 million to 149.4 million in the US in May, recording its first decline in the country. It also said that Facebook witnessed a drop in users in Canada, falling by 1.52 million to 16.6 million.

Inside Facebook noted that the social networking site’s overall growth was lower than normal for the second consecutive month in May. The site gained 11.8 million users in May compared with 13.9 million in April. In contrast, it grew by at least 20 million new users over the typical month in the past 12 months.

“Given that Facebook now reaches 73% of the total U.S. Internet population each month, one thing we should anticipate is that the site’s audience cannot grow forever,” said Andrew Lipsman, senior director, marketing & industry analysis at comScore.

The law of large numbers says that once a site has penetrated the majority of a market, each incremental user becomes that much more difficult to attract, Lipsman said.

comScore’s data also showed that the Facebook’s visitor engagement in the US increased from 4.6 hours to 6.3 hours per month over the past year.

So given its size, Facebook’s future U.S. growth is likely to come more from increasing usage per visitor than its ability to attract new users in perpetuity, Lipsman added.