I cleared FB telephonic round and after 2 weeks I was scheduled with two days virtual onsite interview. Onsite interview comprises of 5 rounds of 45minutes each. For design round 15 minute extra buffer was kept.

Note: I will not be able to share exact questions due to ND I had signed.

DAY - 1:
Round1: Behavioral round, mostly it is non coding round but not a fixed rule hence depends on interviewer. In my case it was full 45minutes discussion on different scenarios and circumstances. He gave some hypothetical situation and I had to mentioned what is the best approach etc.

Round2: Coding round-1, . If one has done enough leetcode will find similar questions if not exact same question. In my case it was finding palindromic substring with some set of operations on given string. Second question was built on top of first question.
I could solve it with in time but there were very less time left for testing with different test cases.

Round3: Coding round-2, It was known question where given city codes and population one has to write custom rand function which will return city code with same probability i.e. higher population will have more probability. There was second question add on to it.
I could solve it in time and validated with different test cases. I could feel interview was satisfied with answers, it went well.

DAY - 2:
Round4: Coding round-3, It was graph problem which I could solve with lot of hints. It took lot of time hence interviewer could not ask second question. I could clearly see he is not happy with my solution which I proposed with his given hint. He was expecting some more optimized approach which I could not mention.

Round5: Design round-4, I was asked to develop an app which can return near by places to the user. I spent some time on requirement gathering. I could propose the interfaces that app will have. I was able to explain over all architecture and execution flow. I got stuck in database related questions and optimizations he was looking for as I don't have database experience. After few tries I mentioned that I don't know how to go further and I felt that is went against my selection.

After 5 days of my last interview round I had been notified that I am not selected. Hiring person mentioned that my solutions were not optimized and design round was not up to the mark hence rejection.

Some learning:

1. All coding interviews are allotted for 45 minutes but actually one will get only 30-35 minutes to complete two questions. Starting 5 minutes goes in Intro and last 5 minutes interviewer reserves for QA.
2. Many places it is suggested that first try to solve the question then go and optimize it. My experience says due to lack of time one will not be able to do so hence start with most optimized solution that you can think of.
3. In design round it is expected that you know all sorts of solutions available even though you have not mentioned any such experience e.g. in my case I was asked what database needs to be used, I mentioned it is read heavy system hence redis as it is in memory DB. I have asked follow up questions in DB which I had no idea. Hence make your self prepare for such question in design round.
4. For behavioral round be prepared with the domain or work you want to do in FB. This is very common question and often one will not be prepared for it. Remember this is not necessary that you will be given that opportunity when you are selected but it shows that you are excited enough to plan something upfront.