I'm inclined to agree with you regarding accountability. But, I dunno, I really don't want my company or business ethics judged by stuff I said to friends in college... through IM. People say stupid stuff all the time.
However, he was talking about his customers, the people using his service. That gives much less leniency, in my book, especially when everything else is taken into account.
I guess I see it much the same as I'd see an investment advisor who open talked about screwing his customers out of their money. FB's financial value comes from the data that Zuck is talking about screwing people with.
I've found that jokes often reveal a lot about what people actually believe.