I had four interviews for the full loop, on three separate days. I did end up rescheduling, because I was sick. Overall, I think I just wasn't prepared enough.

Day 1: 2 coding interviews (2 qs each)
Day 2: Behavioral
Day 3: System Design

Coding Rounds:
They were pretty standard. I don't remember the questions, but they weren't hard at all. I did time complexity and etc, and tbh I was pretty nervous but both interviewers were pretty nice. I was a little dissapointed with my performance because of my nerves but it wasn't a terrible expierence.
Something to note, you don't actually run the code for this round. I was pretty thrown off by that since I did run my code for the phone screening. So just be confident in yourself, and make sure you can talk through it, and step through examples.

Behavioral Round:
I prepared ahead of time for this, but somehow I was still thrown off a little bit. The classic question of "tell me about a time you had a disagreement with a co-worker..." had a bit of a twist....it was "tell me about a time you had a disagreement with a co-worker and then later on you found out you were wrong" with the follow up of, "how did you find out you were wrong?"
Shouldn't be hard, but the example I usually use is how we ended up on common ground, not how I figured out I was totally wrong, so I was a bit thrown.
The rest was relatively ok I think...but so hard to tell if the interviewer liked my answers or not.

Product System Design:
This is where I think it fell apart for me. I really did my best here, but I think it's luck with the interviewer and the question they ask.
Not sure if I can actually share the question, but what I can definitely say is it was unlike any system design question I had seen. It wasn't a hard question, but for product system design, it seems like usually you are trying to show case your API design skills, thinking about how the user would interact with the product....
For my question, there was basically zero user interaction. I think the guy probably was reading a new article beforehand and thought, hm, I wonder how someone would design < redacted >, and then used that as the question for my interview. (think something that the government may do)
The question was really to show off scaling abilities, and data management, which I feel would be more suited for an infra interview instead of product but oh well.
The interviewer really seemed to hate my answers too. To start, I tried to get creative with what the system could do, but he shut down all my ideas when it came to the functional requirements. He only wanted one. And then moving on to non-functional, I only outlined two items before he told me "move on". That sentiment carried on through out the interview. I would try to dive deeper into aspects I was more comfortable with, and he would say "move on".
In the end, my design was pretty lame and lack luster. I did my best, and showcased what little I knew to do, and for that I'm proud of myself. I definitely could have been better prepared, and I own up to that.
Also, I looked up the question afterwards, and literally zero cases of anyone walking through a design remotely like this question.

The result:
I knew after the last round it was going to be a no. It took a week for them to get back to me, which is totally reasonable, but what sucked (imo) is the recruiter called me to tell me. He left a voicemail, saying he got the feedback, and to call him back as soon as possible to discuss.
That kinda sucked because for a second I got my hopes up, which was dumb. What was even more annoying is the recruiter refused to give me any feedback saying it's "against company policy".
Granted, I know the system design was definitely a shortcoming, but would have been nice to know if I was crap at all the other rounds in their eyes.

Oh well. I am starting to think maybe this industry isn't for me, but for now I'm going to keep doing my best :)

(side note: I didn't want to work for Meta anyway....I just really want a job and miss working. I was laid off in a mass layoff wave. Ugh.)

Best of luck to all of you out there!