If Zuck thought Adam was valuable, he would have given him an incentive to stay instead of replacing his position with a newcomer and putting him in a position where he would likely leave the company.
Clearly we don't know if he did. But what we do know is that guys like Adam, Paul Buchteit etc. just want to go do their own thing after a while. And we also know the tendency of founders to sell their co back to their original employer or a close competitor.
It obviously wasn't enough if there were any. And don't you think it is convenient that Zuckerberg "got rid of" the CTO position when Adam left (when he complains about his position not fitting his stature)...and then a couple months later they have another CTO again?
He'll get a new vesting clock with the acquisition and all will be good.