I came onsite for what was described as an hour-long introductory and technical interview. I was greeted by the recruiter and shown around the office (FB NYC), then we chatted for 10-15 minutes. Then an engineer came in and we started the technical part of the interview, which took 45+ minutes (we ran over). First he asked me about Hadoop, since I mentioned that I used it at my current job. I used hadoop for machine learning tasks, so we discussed the details of the system. The rest of the interview was coding on a whiteboard. The questions got progressively more difficult. The first was: Given two string representations of binary numbers (e.g. "1001", "10") write a function that adds them and returns the result as a string as well (e.g. "1011"). The next had several parts: (a) first, write a function to calculate the hamming distance between two binary numbers (b) write a function that takes a list of binary numbers and returns the sum of the hamming distances for each pair (c) the answer I gave for b was O(n^2), I was then tasked with finding a more efficient solution. I struggled mightily, and was eventually helped to the solution by many hints from the interviewer. After that, I had a chance to ask questions of the interviewer, and then we were done. I don't know if I've made it to the next round or not.