It is very, very common to wait until just before an IPO to sue. The strategy is that the defendant will be more willing to settle just to get the lawsuit off the books before the IPO. It usually works against smaller companies.

Google was the recipient of borderline frivolous legal action just before the IPO, but Google didn't bite. In the case of Facebook, I don't expect anything to be resolved pre-IPO, as Facebook has the momentum to deflect nearly anything at this point.