Facebook appears to have seen a small dip in its traffic numbers sometime over the course of February, according to our latest review of data from third-party web measurement firms, with growth returning in March.

Here’s a closer look at what the firms see, along with what Facebook’s own data shows. Each of these data sources has its own means of gathering and analyzing the traffic data — and each can be inaccurate — but they all tend to agree on Facebook traffic trends over time. Looking at them in aggregate can provide a clearer picture than any one in particular.

Compete

Facebook’s US traffic fell from 134 million unique visitors in January to 128 million last month, it reports. But now the site is back up by 4 million to 132 million for March. Of the two nearest competitors, MySpace stayed about flat at 21.3 million uniques and Twitter grew by slightly under 3 million or so to 48 million.

ComScore

After reaching 112 million US users in January, Facebook stayed flat in February — but then it grew again, up 4 million to 116 million uniques in March. MySpace grew by 3 million people, to 58 million. Twitter went from 20 million to 22 million.

ComScore also provides worldwide stats, in which overall Facebook went from 472 million uniques in January, down to 463 million in February, then up to 484 million in March. MySpace and Twitter went on the same roller coaster, seeing drops in February and recoveries in March.

Nielsen

This company is seeing a different trend than its two competitors. Facebook went from 116 million uniques in January to nearly 119 million in February, according to the firm. But then it fell back to 117 million in March. This is in contrast to the February dip seen by Compete and ComScore.

Nielsen has a data-sharing deal with Facebook that might give it extra insight into what is happening in the market. But it’s not clear specifically what data is shared or how it might help Nielsen come up with more accurate measurements.

While the firm doesn’t release public stats for Facebook and its rivals every month, it does show MySpace with 42.1 million and Twitter with 20.1 million unique visitors in March.

Quantcast

We normally also look at this firm’s numbers in comparison with the others, but it hasn’t updated for March yet. The February numbers show Facebook and the other companies roughly level their January numbers, at 125 million, 52 million and 27 million respectively.

Facebook Numbers

We track the company’s self-reported numbers via its advertising tool. Typically, this data is at least several weeks behind. For the purposes of very rough comparison, let’s assume that the data is one month behind. In this case, Facebook itself showed its US traffic coming in at 113 million in February, which is flat from January, then growing to 117 million in March. Note that Facebook counts traffic by “monthly active users” a different term but an equivalent meaning to the “monthly unique visitors” term that the firms use.

ComScore appears to have matched Facebook’s internal numbers most precisely, for US traffic.

Because Facebook only announces official numbers every few months, we don’t have much to compare comScore’s world numbers with at this point. Facebook said it had reached 400 million monthly active users at the beginning of February. It hasn’t updated its official stats since that time. We tracked 436 million users in its advertiser tool for what was likely March. Some Facebook employees have said the company has passed 450 million, as of several weeks ago.

Today, given Facebook’s growth rate, comScore’s 484 million number is still looking high — but plausible. In the past, the firm’s worldwide estimate for Facebook traffic has been tens of millions higher than other available data.

However, we talked to comsScore analyst Andrew Lipsman to get a better understanding of what’s happening. He says that the difference between comScore’s numbers and Facebook’s internal numbers may come from comScore counting people who visit Facebook to look at a Page or a publicly-visible profile, but who are not members of the site.

In other words, it might not be too long until Facebook announces that it has half a billion monthly active users around the world — and third party analysts may beat it to the punch.