Three third-party measurement services agreed on United States Facebook traffic in May, at least directionally. They each showed the company making a net gain in monthly unique visitors to help make up for what has been a slower year by each of their accounts. Beyond this initial point of agreement, the data diverges, as you can see in the chart below — that is, except for a few main points.

Facebook is still posting overall user gains in the US if you look past month-to-month changes, but it is doing so more slowly than it has in previous years as it nears saturation in the market.

Nielsen published monthly numbers yesterday showing that Facebook added 6.28 million monthly unique visitors in May to reach 140.3 million, following a loss of 1.64 million in April, a gain of 4.92 million March, and a loss of 4.84 million in February.

Compete showed growth of 4.75 million unique visitors in May for a total of 142.7 million unique visitors, following a loss of 2.82 million in April, a gain of 6.66 million in March, and a gain of 351,000 in February.

ComScore, meanwhile, had indicated net growth of 3.16 million to 157.2 million uniques, following a gain of 1.10 million in April, another gain of 2.30 million in March, and a loss of 2.34 million in February.

In contrast to most of the past four months shown by the others, our Inside Facebook Gold service, which tracks Facebook traffic via its ad tool, showed a loss of 5.88 million in May to end at 149.3 million, following a gain of 260,000 in April, another gain of 2.78 million in March, and an additional gain of 3.32 million in February. See our previous two posts on the matter here and here.

If you look further back (see above graph), you can observe what looks like a general deceleration since last fall, with each service reporting losses in at least two months — except for Inside Facebook Gold, which only showed a loss in May.

Zooming out, there are 307 million people in the US, and nearly 240 million of them use the internet, at least by the official numbers. Going by comScore’s totals, Facebook is currently reaching 72.3% of the 216 million unique visitors for May. You could say that Facebook has 60-plus million users to go if it’s going to reach complete saturation (depending on whose numbers you look at). But going even close to there would be a first for the internet.

The reason is that Facebook is already the second-largest internet brand in the country, behind Google’s 155 million monthly uniques, according to the latest Nielsen data. In looking at Facebook’s efforts to continue innovating its core product — adding more features like Places and Deals, taking away the friction of page-loads to access parts of the site like Photos, going further into mobile with new apps, and generally trying to make itself easier to access and more available anywhere — one can see how it is working to get there.

The result has been 50% of of users returning every day, according to Facebook, with third parties generally showing similar engagement numbers. It’s especially notable that the company has been self-reporting this percentage since its early days, suggesting that the site doesn’t seem to have gotten less popular with most existing users as it has grown.

So with that in mind, it’s not surprising to see some months come in lower than others, or even dip into negative territory this year. We expect these rises and falls to be much more common going forward versus the breakneck growth in 2008, 2009 and the first part of 2010.

Which brings us back to how to interpret month-to-month data from these various sources. It is obvious that they all contradict each other. They have varying methodologies (and bugs, as we noted about the ad tool in previous posts), and there are also seasonal factors like college graduations that affect each piece of data. No two sources has ever exactly agreed on growth or total numbers, although comScore and Inside Facebook Gold tend to match up more often, with the others tending towards each other as well.

Therefore, as we say in the posts we write every month about all this data, do not jump to conclusions either about increases or decreases in Facebook user numbers in the short-term, because it is its long-term trajectory that shows its current health and future potential.