A facebook xss vuln is probably not enough to do damage. However, a serious 0day or attack plan that could take down the network would be valuable now that it floats on a public exchange. During April 20 - June 20, 2011 sony shares fell from $30.14 to $24.28 during the PSN hack. A vuln that could take down facebook could allow shortsellers to profit if it was severe in magnitude even if not in duration especially if it created FUD about Facebook security.