Wonderful people, fun work, great culture
Pros
Comp, benefits, and perks are incredible. Company culture is so, so good. I've had two great managers who both have done an excellent job supporting my growth and guiding my career. It also feels like senior and C-level management really cares about employees. It feels like Facebook the company really tries to support me as a whole person (but be sure to ask for what you need! nobody's a mind reader). For eng, bootcamp and team selection are such a good idea. I freaking love my team, and I get to own my decision to join that team (rather than some hiring/allocation manager deciding where I get to go). Very few operational rules/bureaucracy. I can ssh into any machine I need to. If someone's fighting a fire anywhere in the company and I am able to help, I feel like I can always jump in to help. I really like Infra, so being at a big company like FB is super fun.
Cons
Oncalls can kill your sleep during your oncall shift if things are noisy overnight There's a little bit of an "always on-the-grid" culture which contributes towards the 4/5 work/life balance, but it's generally not too hard to fight it. Most people who work a lot end up doing it because they're passionate about their work, anyway.