I was referred by an employee and was contacted by a hr recruiter, who scheduled a phone interview with me. From reading online I was expecting a basic screen with questions like why do I want to work at Facebook, and what I do currently. I wasn't expecting any technical questions at this stage but she did throw a basic search question, which flustered me a bit, and after a few minutes of me talking it out but not giving the exact right answer, she said she would get back to me with next steps which would either be to complete one of the puzzles on the website or to schedule phone interviews with an engineer. Promptly the next day she responded that I needed to do at least one of the puzzles and that there was no time limit, but the faster I completed it, the faster I could move on. I'm currently working on one of the puzzles.