I don't have quick access to user numbers, but Google search trends is interesting (especially when you consider the readwriteweb example today of people typing 'facebook login' into google).

Here's the comparison of Facebook, Myspace, and Orkut - http://www.google.com/trends?q=facebook%2C+myspace%2C+orkut&...

Nothing surprising, except that there are some areas (USA? Tijuana??) where Myspace still out searches Facebook.