Pros
There are a lot of very smart people working at Facebook and this makes it a great place to work if you enjoy learning. There are regular presentations and tutorials from some of the best in the business.
The work is challenging and once you have proved yourself there is also good opportunity to move to other positions and projects. In fact, there is an engineering career track right up to the top levels so there is no pressure for dedicated engineers to turn towards management in order to get pay increases.
Add to this all the other perks such as 3 meals a day, laundry service, shuttle service, etc.. and this has to be one of the tops jobs in the industry.
Cons
The baseline expectations are high so this is not the kind of place for someone who does not love their work.