The interview was at Palo Alto last Friday and got a mail from the recruiter yesterday, so it took 5 days to get the outcome.

There were total 4 rounds as the typical SWE interview at FB.

1. Coding: Leetcode Medium
2. System Design
3. Behavioral
4. Coding: Leetcode Easy

At first, this was my second interview at FB. Trust me, for coding preparation for FB, just Leetcode premium is the answer.
Coding 1 was the exact same question of FAQ of FB. My recruiter said Coding 1 and behavioral was okay.
But the System design and Coding 2 was negative.
My background is embedded and 3G/4G protocol, how can I answer well on designing web based system? (It's one of Grokking... question but I never expected they would ask it)
Of course I tried to answer as much as possible and drew the block diagram, but she kept asking me on server side - what kind of DB needs to be used, what is the DB capacity supposed to be etc. As I know, the interviewers are selected by recruiter at FB, but I strongly complain they have to differentiate the system design question depends on the interviewee's background.
And, the second coding question... I had prepared around over 300 Leetcode question including many HARD ones, but I kind of failed on this EASY question, it's nonsense. I can't say the exact question but it's also at Leetcode but less percentage asked, that's why I didn't pay attention. I didn't do totally wrong, I just suggested a different solution which works but the interviewer asked not to use that way. So how can I do? They asked me the second question - I'm not sure if this is also in Leetcode but it's really easy - and I wrote the code right away and they finished the coding, not by time up. But I didn't put the corner condition check here. I think that might be the negative point. So do not finish the coding even the interviewer declared it's finished until you complete the coding has no minor issue.

I will keep trying FB, because I convince their coding questions are definitely from Leetcode.