I would say that at the very least, we should give Carmack the benefit of the doubt. He has proven to be a visionary and able to execute on all manner of awesome tech for many years.

If he leaves shortly after this acquisition, then we know something is amiss. If he stays, then it should be for a very valid reason - i.e. that they can probably do much more inside of FB than outside.

Keep in mind he was independently wealthy before, so it's likely that he wasn't as tempted by money as say the founders were (assuming that they weren't independently wealthy - which I don't know if they are).