His low point was when Yahoo offered to buy FB for a billion dollars. Dark times.
That being said, there is something to be said about how this comment was made. Because the hard part wasn't turning down the offer. It wasn't even dealing with the outcome of an unpopular choice per se -- hard to imagine these people thought "acquired by Yahoo" was the road to product success, even back then. It was dealing with the outcome of telling people they'd have to wait, possibly forever, to see their efforts rewarded (it took them 6 more years to IPO, with Zuckerberg famously devaluing the stock of early contributors along the way).
Noticing the lack of trust (possibly in both directions) _is_ a dark time, but it's hard to come to that conclusion when these things are referenced so obliquely ('people left' isn't a dark time, it's business).
That's more or less the same situation that the TV show Silicon Valley used to stress out their founder. He's got something with potential and an offer to buy it for more money than he's ever thought about before. What do you do? Maybe go throw up in the bushes? How would it feel when you make your decision and then most of your team leaves? What if they're right and you ruined everything?
That scenario is dramatic enough when you get to watch it happen to someone else. Living through it could easily be "dark."
HN users sometimes feel that the rules are off when a rich and famous person is the target, but that misunderstands why we have the rules. It's not to protect the rich and famous, who (as cynics are fond of pointing out) don't need it, except of course insofar as they're human beings like anyone else. Rather, it's to protect the community from becoming a mean place that only mean people would want to be part of, and eventually not even them.