Talent, commitment, autonomy, impact.
Pros
The most important reason to work at Facebook is the other people working at Facebook. Almost all of them are very, very good. Down to about the 20th percentile of engineer, you can pretty much take their word as gold: if they say they'll do X in time Y, they will, and often a better X in a little less time than Y. This is a kind of super-power: it reduces the overhead of coordinating complex projects significantly. Facebook is also a mission-focused organization, in the same sense that the military, or a relief agency is. Most employees understand the goal of the company to be larger than just making its employees and shareholders materially successful. You will enjoy a vertiginous amount of authority as a front-line employee at Facebook. For better and for worse, on your first day at Facebook, you're given the keys to the proverbial Ferrari; you can push code, fix bugs, implement features, and yes, crash the site as much as any other engineer. Many user-facing decisions end up getting made by individual engineers. This also implies that your work will have impact. Almost everyone you meet in your daily life is a user of your software. If they are unhappy users, it is your fault and responsibility to make it better. If they are happy users, you can enjoy your share of the credit. All the basics (salary, equity, career advancement, food, miscellaneous perks like shuttles from SF) are world-class as well.
Cons
A mission-oriented job is not for everybody. If you are looking for a paycheck, and a ton of time to pursue your model railroading hobby, Facebook is going to be a weird fit for you. This is not to say that people don't have hobbies, or that everybody works 70 hrs/wk; but if you are completely "checked out" from your job, this will not be a happy place for you, not matter how good you are at it. The physical work environment (open plan office, graffiti on the walls, lots of free-form discussion and collaboration) can be jarring if you're coming from a more typical corporate engineering environment.
