The number of Facebook users in the United States has dropped by just over 1.1 percent this year according to one analyst. It’s not necessarily cause for major panic, but the theory that there’s an inherent limit to market penetration is starting to look more credible.

The report comes from Capstone Investments and covers changes for the past six months. It’s important to remember that Facebook itself only announces milestone figures (and could be on the verge of announcing the billionth user in the next couple of months), so the figures in the report are merely an estimate. While there’s a lot of third-party objective data to work from, a 1.1 percent shift either way could easily be a statistical blip rather than a sign of a genuine trend.

The figures appear to be for the total number of registered users rather than the number who are regularly active on the site. If that’s the case, a decline would be more serious as it means those who are canceling accounts outright (which isn’t a simple process) are outweighing new members. In theory at least, you’d also expect children becoming old enough to want an account for the first time to outnumber older users who are dying, particularly as canceling an account isn’t exactly number one on the list of priorities for bereaved people.

At the very least though, it seems fair to conclude US membership is no longer booming, a pattern that’s emerged since the site hit the point where 50 percent of Americans had signed up. Capstone’s report suggests that may be no coincidence. It examined 24 countries with at least a 50 percent penetration rate and said only nine continued to show solid signs of Facebook membership growing.

To some extent this may be inevitable. Roughly 20 percent of the US population is aged 14 or under, and officially pre-teens aren’t allowed on the site. Meanwhile around 20 percent of adults aren’t online at all. Crunch those numbers together and you’ve got around a third of the population who Facebook can forget about reaching right now.

Put another way, Facebook has already signed up three out of every four potential members. It’s certainly true that many of those who’ve refused to sign up have specific reasons for rejecting it, such as privacy concerns. However, the main reason may simply be a reversal of the factors that caused Facebook to boom.

Once Facebook membership hit a critical mass, people signed up because they already knew lots of people who signed up, and the network spread. It may be we’ve now reached the point where there are offline networks of people who know few people on Facebook, and in turn mainly have real life friends or colleagues who also know few people on the site.