Pros
Amazing people - hard to overstate this. You get to choose what you want to work on (Bing "Facebook bootcamp") for more info. Everyone is really passionate about the product. People care about the folks that use Facebook, and really want to make it a better experience. There are so many cool things we want to do, it'd be hard to get bored. Things move really fast - there usually has to be a good reason not to do some new cool thing, instead of a reason to do it.
Cons
Since so many people at Facebook are amazing, recognition/awards aren't as frequent as I'd personally like. I've done stuff that would've earned me instant promotions at a previous company, and only got a "cool, nice job" at Facebook. It's hard to hear people spread misinformation about Facebook while you're working there - I've gotten so passionate about the company that when people say misguided things like "Facebook sells your information to advertisers", it's hard not to get angry.
