It is a limit of 500 shareholders, Perrine. Yes, there is considerable speculation that they are repackaging the deals through GS in order to skirt the issue.
In terms of value, $50 billion is much too high, but I would put it higher than the $1.5 billion stated in Forbes last year. Myspace is laying off half of their workforce, and other serious (western) competitors are not posing much of a threat at this point. It's certainly a story to follow, especially if Zuckerberg decides he's had enough and considers an IPO.