Facebook has an incredibly high hiring bar. I never once met someone who I didn't feel was qualified to be there. I was seldom the smartest person in the room.
The high average engineer quality allows the company to give developers almost complete autonomy. At Facebook, you not only get to choose your team, but once you're on that team, you, not some PM or manager, decide what to work on. You're evaluated on impact twice a year, and Facebook has an expansive and nuanced understanding of "impact" that rewards things like developer productivity improvements, side projects, and removal of bad code.
Facebook encourages cross-team collaboration in a way that Google only dreams of doing. There's a single codebase unified under "fbsource", of course, but also a lack of OWNERS files. That means that it's your job as an engineer to decide what code to add, not some team of blessed approvers. There's no readability process. Teams are trained to expect people from all over the company to contribute code. It's a dream job if you want to wear lots of different hats, or if you have a maniacal obsession with following a problem to its root cause and fixing it.
One engineer famously traced a sporadic failure to a single bad register on a single core on a single server. He was recognized for it too: Facebook has a "fix of the week" program that highlights heroic fixes and clever hacks.
Facebook management "gets it" in a way I haven't seen anywhere else. Developer productivity is paramount. When something goes wrong, management doesn't overreact. The general ethos is to apply tooling to make developers better, not to add process to make developers slower.
Best of all, Facebook is fun. The code review tool, Phabricator, has built-in meme support! The internal Facebook groups are full of interesting discussion and trolling (of the good sort). The people there all feel like, well, characters. They're memorable in a way I haven't seen at other technology companies.
Best of all, at Facebook, there's none of the insufferable technical grandstanding you see at other companies. It's hard to describe --- at other companies, people with just enough competence to be annoying regularly create elaborate word-salad design documents that would fit right in at /r/iamverysmart. At Facebook? People say what needs to be said.
The most striking thing about Facebook is how it does more with fewer developers. At Facebook, you feel incredibly productive. There's always enough to do. Teams are much smaller than equivalent teams at other companies, but somehow move faster. Management trusts you --- I never once heard "you can't check that in: the technical risk is too great".
Oh, and the pay is fantastic, especially if you demonstrate extraordinary impact.
Is Facebook perfect? Of course not. After all, I'm not there anymore. Some people ragequit. The company is growing more corporate over time, little by little. Traffic in MPK is nasty. Traffic in SEA is worse. The tooling has some room for improvement --- but you're welcome to send patches! The playfulness isn't for everyone. The open allocation policy sometimes leads to duplication of work, forcing people to discard their hard work. There are few hard rules, and sometimes you don't know where you stand in the power structure.
All that said, I'd go back to Facebook in a millisecond if circumstances lined up. Despite Facebook's flaws and its inevitable slow decline, it's still the best fucking engineering culture on earth right now.