I was contacted by the recruiter on LinkedIn followed by a phone screen. I have worked for like 9 years before I took a break for childcare. Now trying to get back to work. Unfortunately its pandemic time. Here is my experience with FB interview.
One thing though the recruitement team is very good and they communicate really well.

Phone screen:
It was 2 medium questions from leetcode.

1. https://leetcode.com/problems/minimum-remove-to-make-valid-parentheses/
2. given two array perferming union and intersection.
   I answered both and discussed time complexity. You have to keep communicating and also coming up with testcase very important.

I cleared so they set up a virtual onsite. 5 rounds 3 coding, 1 product architecture and 1 behavioral.

Onsite

Round 1:

• https://leetcode.com/problems/is-graph-bipartite/ which I was able to do it after discussing the solution. He wanted me to trace the solutions for 4 different testcase. I did. We discussed time and space complexity. He was happy and he said he had planned to ask only 1 question as we were going to trace the code for testcases.

Round 2:

• Behavioral normal situation questions, went pretty well. I had practiced and written all my previous projects. So it went well. Thats what I think.

Round 3:

• Sum to 100 I took little time and learnt that order of the numbers should not be changed. So I gave a backtracking solution. But I could not complete the code. I think this interview is what costed me.

Round 4:

• https://leetcode.com/problems/product-of-array-except-self/ I discussed this and gave solution and wrote code. She was happy with solution.
• https://leetcode.com/problems/kth-largest-element-in-an-array/ I discussed 3 approaches. One sort which is n (log n), use heap but this will have space complexity, 3rd approach quick select with random number. She asked me about worst case time complexity I said n^2 in first and last but heap it will be same as average. She said if i am confident to write quick select implementation, go ahead and write code. I did I was running out of time as there were few audio issues so we had to repeat. I could write the code. we discussed time compexity and how, heap might give better time complexity in worst case but at O(n) space.

Round 5:

• Design API- newsfeed we did on coderpad. But He was pretty happy with my solution.

After interview I sent thank you email to recruiter. He said he will get back to me once he gets all fedback. Next day he again sent email saying he has got all feedback waiting on one piece. This was friday. Again Monday he sent email saying got all feedback sending for review. Tuesday he said we have decided not to proceed.

I felt it went pretty well except that one coding interview. Don't know what else might be reason. But had wonderful experience. FB has best recruitement team. Only thing I feel is due to virtual set up 5 minutes buffer have to be provided, as my last 2 interviews I had audio issues.
I was worried about the one coding interview. But when recruiter sent me daily updates proactively, I felt may be this is leaning towards positive. So don't know what happens in review process.

Communication is key. What ever you are thinking, think it loud. Hope this helps all the best!