Turns out there is a LOT about ads at FB. Not everything, but a lot. Particularly in the Seattle office.
I was surprised I hadn't passed the interview (thought I did well), but in retrospect I'm glad. Whether that's sour grapes on my part, the fact the news has been full of reasons to be glad not to work at FB, or that I was very concerned that anything I did would end up pushing ads is something I can't be 100% certain of.