Earlier quoted context omitted.
To all hackers: If you speak perfect english, please proceed to submit exploits to the FB security team and collect $5k on your way out. If not, post to blackhat forums and receive $10k. Or use the exploit to manipulate FB stock price (I'm sure posting as Mark would move the stock price significantly, if only for a few minutes. But that's enough to earn a lot more than $5k).
He posted on Mark's wall, not as him. Also, relative to other places you could be spending your time, FB security issues yield relatively little in the blackhat market because of their highly responsive abuse and security teams.
Sure this bug was like that. But what if someone discovers a bug that allows you to post as the person?