> Only got through one question, and even that required assistance from the interviewer. My understanding is that if you don't finish two problems, you're doomed.

Different loops and interviewers conduct interviews in different ways. There is no "one problem" rule.

Usually the questions are structured in layers where there's many follow-ups available. Based on the question, the interviewer, the loop, they can ask one problem or more. Unless you are the interviewer, it's not clear how far into the question you've been and what the expectations were. It is common to ask two questions though.

Context: I have 300+ interviews at Facebook and been active in the whole process.