The engineer's computer was compromised using a real zero-day exploit targeting an undisclosed piece of software. What the diddly ding dong is Facebook doing with real 0-day exploits (besides using them in fire drills)? More importantly HOW did they get their hands on 0-day exploits? And what other exploits do they have/buy/finagle? Is it on a regular basis?
(I work at Facebook but don't know anything about the event in question or if this post is accurate) I suspect it wasn't actually a "0-day" in that sense, but rather a disclosed but unpatched vulnerability, and described as "a real 0-day exploit" in the article because of the typical reduced fidelity of press articles.
So then next question, how come the vulnerability was unpatched?