I've been a hiring manager in the past. A company uses referral bonuses to incentivize staff to bring in the top talent into the company.
If you are really not motivated by the referral bonus (eg your startup was aquired and you're not short of cash these days) then you shouldn't take it - the start up could use the valuable cash elsewhere. I guess burn isn't a big problem for Facebook, but then to your point in your comment, Facebook is still a big startup isn't it? (is it?)
I also think its poor to offer image on the interview questions/answers (yes, even if they are on a public site somewhere). If I was the hiring manager I would be upset by that as it skews my ability to interview all candidates equally (ie the ones who did see the interview questions and the ones that did not).
I don't think you are coming from a bad place or doing anything under-hand but I think it isn't the right and professional way of obtaining talent.