When it comes to Facebook’s traffic in the United States, analytics firm Compete has an interesting record. The company has released its November numbers for web sites in the US, and here’s a look at what it shows happening to Facebook now.

The service actually lost around half a million monthly active users in November, to end at 128.3 million. Compete previously showed that Facebook’s growth was flat for much of the late spring and early summer, then grew from 122.2 million at the end of July to 128.9 million by the end of October. This does not clearly fit with the other data points that we have available for Facebook’s growth.

First of all, Facebook’s own advertising tool shows that Facebook had 98.1 million users in the US at the end of November, an increase of 4.1 million from October. Beyond the discrepancy in monthly growth numbers, however, that overall usage number is drastically lower than Compete’s estimate. One reason for this might be that Compete is counting people who use Facebook Connect on other sites every month, but do not go to Facebook.com itself — although it’s not clear how large this group of people actually is.

ComScore and Quantcast, two other services we regularly look at to better understand Facebook’s traffic, have not released their monthly numbers for November yet. But in terms of Facebook’s US traffic over the last half a year or so, Quantcast has showed some of the same slow growth that Compete does from earlier this year, while comScore shows ongoing growth starting in February. Overall, Quantcast had Facebook at 104 million US monthly visitors at the end of October, with comScore estimating 97.4 million.

So, we’ll wait and see what these two firms have to say about Facebook’s November traffic before jumping to any conclusions.

In the meantime, it’s also worth nothing that Compete shows the two other most popular social web services in the US — MySpace and Twitter — also not growing. Apparently, according to Compete, US users are feeling a little tired of socializing on big web sites.