I believe this will hinge on the details of how Facebook represented the value of the securities to ConnectU. Representations of the value of a security are legally perilous. If Facebook's lawyers at the time were good, I imagine Zuckerberg will get a pass. If they missed the dots on a few "i"s or the crosses on a few "t"s, then Zuckerberg could be in for a long ugly ride.
I am not a lawyer, but I worked in the securities industry in a regulatory supervisory capacity (series 24, plus a mess of others).
Good general advice: Any time you feel the urge to tell somebody what your stock is worth, avoid the urge unless your legal department is involved.