This looks weak. They knew what they were getting. They agreed to take it. And now they've changed their minds. The secondary value of that stock is higher than the value at the time of the Microsoft deal (http://www.sharespost.com/companies/facebook), so they're basically suing because Zuckerberg accidentally gave them a bargain.
If FB can buy back that stock at the original valuation, they'll get a good deal.
If FB can buy back that stock at the original valuation, they'll get a good deal.