Ideas are worthless. Undoubtedly he took their concept, but I sincerely doubt he developed a complete copy of their idea. Whether or not their idea would have worked is up for debate. But it's a pointless debate, because ideas are worthless... Unless of course, they are patented.
Knowing them, knowing him, having been there at the time, I can only confirm that they designed the entire rise of Facebook using the principle of exclusivity to build interest in users 1) outside Harvard, 2) outside the ivies, 3) outside colleges, etc. Zuckerberg was right (edit: correctly saw he was able, not morally right)toun with their ideas, and deserves credit for executing them to perfection.
At the time I thought they were foolish for not having a contract and for not having built up enough technical skill to understand that Zuckerberg was taking them for a ride. This is definitely true, but they corrected the problem and have recovered brilliantly.
That's the risk you take in software in general. What stopped Instagram from copying Snapchat? Nothing. Micrsoft used to snuff out competitors in the office suite space all the time. MSN copied AIM. IE copied Netscape. It happens. You either work with founders you can trust or you don't.
And its not just with start ups. People who are more closer to work, generally tend to maximize returns for themselves above those are who are away from it.
This is true even in Big companies. Most product managers I know barely contribute anything to the product, most of the times its the engineers doing all the work and the product manager is just there to provide the validation 'This looks fine to me'.
There is a good reason why carpenters, plumbers, drivers, <skilled_worker> generally do better on the longer run than the supervisor ever does.
What if your game idea is half-baked, incomplete, and generally kinda crappy, the engineers realize you don't have a complete idea, but then go on and develop a fully-baked similar idea on their own later, and it becomes a hit?
Let's add some ambiguity to our "poor genius"/"brilliant thief" scenario :)
