you mentioned questions on scalability et al, answers to those questions are very subjective. I went through a similar experience at one of the biggest tech giants out there ... went something like this - i was in the final face to face interviews and did well in all of those IMHO. It was a big interview day at the company and lot of candidates flew in all over the globe. So all interviews done, we went for lunch in the cafeteria, and before I started I was called for another round of interviews (and couple of other guys were called too). To be honest I hate lunch interviews, I think its cruel! But anyways, I was asked about design and scalability etc. and that was the last interview that day. After a few days I get a rejection email from the HR folks. I was in the same boat as you are today ...