The Facebook interview process can be quite lengthy as it involves several people and steps.
I applied through an employee referral for 2 different roles, and heard back about one of them quite quickly. I went through the process again for the 2nd role after the 1st was completed.
I had a preliminary phone screening with the recruiter, then a phone screening with the hiring manager and 4 onsite interviews on both occasions.
Onsite interviews are with both management and peers (from the same team or other team).
I had an extremely positive experience with both recruiters. They both came across as present and committed to the candidates success. They were proactive and provided targeted input and feedback. Even after the interview outcomes, they both took the time to make a phone call to provide feedback on the interviews.
There is no formula for the onsite interviews, all of the interviewers have different styles.Some of the interviewers came across as not fully calibrated. Overall the experience was positive in both occasions, it is not easy to impress so many different people, with such different styles.