MySpace is running out of surveys that still place it as the #1 social network in the US. After yielding the position to Facebook earlier this month in stats released by Compete, today, the social network slips to second place in the latest numbers from Nielsen Online.

According to data from January, Facebook saw 62.4 million unique visitors, besting MySpace and its 60.6 million uniques for the first time in the survey. At the end of 2008, MySpace still held a narrow lead, with 58.4 million unique visitors in December compared to Facebook’s 55.2 million.

As for those engagement numbers that MySpace likes to cling to? It can no longer claim a lead there either – engagement fell to one hour, 35 minutes per user in January, down 32 percent from last year and barely more than half of Facebook’s two hours, 50 minutes per user.

We’ve been documenting the ascent of Facebook versus MySpace on a monthly (if not more frequent) basis, but for some longer-term perspective, check out this post from mid-2007 to see just how big a shift this represents. Back then, MySpace claimed 3x more traffic, 3x more engagement, and claimed to not only be America’s most popular social network, but its most popular website.