Pros
If you're the type of go-getter who does does their own thing by force of sheer will, you're given a lot of leeway to try new things. Some nice people used to work there.
Cons
Multiple founders had different visions for the company, leading to constant chaos and shifting priorities. There were problems with sexism and sexual harassment. Constant layoffs. The company just dives into whatever the current trend being pushed by Facebook is, but Facebook is not a fair or reliable partner. Total chaos all the time. Claims of a politics-free culture just masks the fact that it's a dog-eat-dog mess. Company was in the habit of exploiting a vast free volunteer workforce of underprivileged high schoolers and college students, treating them like employees and offering nothing in return.