Last month I wrote that for the first time ever, Compete reported Facebook traffic to be greater than MySpace’s traffic in the United States. This month the company is reporting that Facebook attracted 10 million more visitors than MySpace in January. Are Compete’s numbers really far off? According to numbers that Mike Arrington published from comScore, MySpace attracted 20 million more users than Facebook last month.

So which numbers are more accurate? It’s difficult to say. Compete combines ISP data with a large panel of users that have installed the Compete.com toolbar. comScore is widely accepted as the standard for online metrics. They arguably have the largest panel of users voluntarily submitting complete information about their internet usage.

Ultimately there’s no way to say exactly when Facebook will surpass MySpace but most experts have been touting comScore’s numbers as most accurate. Bloggers have been using Compete’s numbers for a couple years now, as they are traditionally the most accurate free data. Quantcast, another public reporting site, suggests that MySpace is still ahead of Facebook but the gap is extremely narrow.

The latest data suggests that MySpace is ahead of Facebook by 2.3 million users, a much smaller gap than comScore reports, and much closer to the numbers reported by Compete. At the end of the day it’s really hard to determine which numbers are most accurate but based on all the publicly available numbers, I’d guess that Facebook will surpass MySpace’s domestic traffic well before the end of the year.

Do you have any guesses about when Facebook will be larger than MySpace domestically?