The onsite interview process was fair, and the difficulty level was average. The first person to interview me had horrible communication skills and couldn't explain the coding problem clearly.

The remaining three interviewers were very good in communication.

After the on-site interview, Facebook requested yet another coding phone interview.

This post-onsite phone interview is what went wrong for me. The interviewer had poor interview skills. They asked me an advanced algorithm question about graph/tree traversal (search for "same fringe problem concurrency" to understand). I provided a natural solution that was not space-efficient, but the interviewer was looking for a specific solution they had in mind.

I lost the opportunity because of this question.

I would give thumbs up to the recruiters handling the interview process; I believe they are among the best in the industry.

Same fringe problem for non-binary trees.