The biggest risk within close reach is that ConnectU might get the appeals court here to reverse the judgment confirming the settlement, with resulting uncertainty that would prove hugely disruptive to FB.
In other words, whether or not ConnectU ultimately has a winning case, the doubt that would be cast by its having a case that might even block FB from doing business down the road would hang over FB like a "cloud on title" does over real property. Such a cloud taints the underlying property because no buyer will touch it until it is cleared. This would affect FB's ability to get future funding and would most certainly mar any IPO it might be contemplating.
That is why FB will fight the battle fiercely at this level. Everything is clean so long as it can make the settlement stick. If not, everything busts wide open. Of course, FB would not allow that to happen and would pay a lot of money to prevent such an outcome. The ConnectU founders know this and are essentially pushing to magnify their payout by bringing that cloud into play.
Actually, that is the major risk here. As to other risks, such as a risk of being found to have committed securities fraud, I just don't see that as a material risk here. The real problem, and it is huge if this gets re-opened, is the threat of a trial over trade secret misappropriation, and that is immediately opened up if the judgment is set aside. The underlying case could drag on for years before getting to trial but the "gumming up" effect of its pendency will mar FB's prospects throughout that entire period.
FB might also face legal action by its investors but the far greater likelihood would be that they would pressure FB to settle this up at whatever cost, perhaps at the expense of some of Mr. Zuckerberg's personal holdings in the company. So he would also be under some substantial and immediate pressure should the judgment be reversed.