It looks like they sued in May 2014, and the acquisition closed in July 2014, so it looks like facebook just didn't do adequate due diligence.
I guess it's possible they just didn't care. What's another half billion dollars to facebook?
The interesting question is whether FB will indemnify Carmack. $150M is a hell of a speeding ticket.
But a lot of the negative posturing from ZeniMax during the case still rubs me the wrong way.