Pros
The people are amazing - passionate, competent, quirky and interesting. The perks are fantastic. The rapid growth makes it a great place to learn, since you're constantly solving new technical and social problems. You really can own your own impact and find the best projects for yourself; this requires being able to argue for your point of view, but learning how to do that is a great opportunity in itself. The company really does care about the users, and has a fundamentally positive impact on the world.
Cons
The office is crazy crowded, which can make it hard to focus. Hiring lots of new grads means a lot of time is spent answering questions and trying to train the recruits. Also, the interview load is pretty heavy. People tend to work A LOT, so it can be hard to define boundaries and disconnect.