Working at Facebook wasn't a positive experience. You'll definitely meet some great people amongst your peers (lovely, smart people), but you'll almost certainly meet some awful managers too (at best, ineffectual & negative; at worst, toxic). The managers I experienced talked to the team disrespectfully, as if we were children, on a regular basis and always in a negative way, never encouraging. New people to the team were always shocked initially. The 'heightened candidate experience' that they insist on for hiring, is out the door once you're actually on board and I found it a cut-throat, toxic environment, where everyone is watching their back.
They are very heavy on KPIs to an agency-level.
The scope of the job is also very narrow due to the size of the org, so you very quickly find yourself bored and although I undoubtedly was learning some new skills, I was losing some I valued that I had acquired in previous roles.
They don't live out the values they profess, or that are reflected in the benefits package, so you might find yourself unsupported through difficult, personal times and wondering if all the great benefits are worth feeling so demeaned. They're not.
