Facebook’s US traffic growth had a big finish last year, according to web measurement companies comScore, Quantcast, Compete and Nielsen. For the month of December, each of these firms showed growth in the millions of new monthly users from November. Here’s a closer look at each one, plus data on Facebook’s international traffic.

ComScore:

Facebook gained 9 million monthly unique visitors during the month, to reach 112 million monthly uniques in total (double where it was at a year earlier). Meanwhile, social network competitor MySpace continued to drift down and microblogging service saw slow growth.

Quantcast:

While neither Facebook nor MySpace and Twitter are directly measured by it, Quantcast shows similar trajectories. Facebook grew to 117 million in the US, while MySpace and Twitter gradually fell.

Compete:

Meanwhile, Facebook grew by around 4 million new users to 132 million monthly unique visitors, as we covered when Compete released December stats earlier this month.

Nielsen:

The company reports nearly 110 million unique visitors, close to what comScore has.

Facebook’s own advertising tool shows the service reaching nearly 103 million monthly active users, with a gain of 4 million. Why is this number lower? One reason, it appears, is that the advertising tool shows data from more than a month prior.

Each measurement service has different methodologies — Compete, for example, may somehow measuring Facebook Connect integrations on other sites, thereby pushing its numbers higher than the others. But the overall trend is pretty clear in terms of US Facebook traffic: December was a big month, concluding a big year.

Worldwide, we only have data from comScore and the advertising tool. But the numbers are even higher. Comscore shows a monthly increase in December of nearly 31 million, reaching a whopping 469 million monthly unique visitors. Facebook announced it had grown to 350 million monthly active users at the start of December; Facebook’s advertising tool showed that it had grown from 337 million in November to slightly more than 350 million by the end of December, according to our Global Monitor. ComScore’s international numbers may be high due to more general difficulties in measuring traffic in some parts of the world.

ComScore’s Andrew Lipman also took a closer look at the numbers in a blog post yesterday. From his post: