Google Associate PM interview questions

The interviewer was 15 min late for the interview. The same thing happened in my first interview with Google (this is round 2 questions I am sharing). It started off with a) Tell me about yourself b) How much money does Facebook make c) Design an elevator for a building (no specifics on what kind of building). Follow up design with two elevators and etc. I didn't get the job because I think I messed up in question b. I could only think of one way Facebook generates revenue (advertising), I forgot about the cutesy hearts and stuff you can buy as another source of revenue. But I couldn't figure out how to come up with how much they make per year accept rambled on how much each customer would pay and depended on number of page impressions, size, format and frequency of add displayed and to get the total...I mentioned that I know how much reported net worth of Owner of FB is so it has to be x much more to cover cost of IT, resource, etc. I have been interviewed by Google, Apple and Microsoft and to compare all of the interviews I have to say Google's was the least structured. It almost felt like they came up with a question but did not come prepared on who the customer base (question c) which is highly important for a PM to know...