Pros
I've been in this field (recruiting) for quite some time...I've worked for start-ups and large, F50 companies and Facebook is by far the best place I've ever worked. This is one of the only companies I've worked for were you live and breathe the mission/values and you can see how they are a part of your daily life. In my team, I'm treated like a professional and given the freedom to work with my clients the best way for me to have the most impact. I'm not going to speak for eveyone, but work-life balance is here for me. The benefits and compensation are ultra-competitive and I feel the company really listens to our needs. I trust in Mark and the rest of the leadership team to put Facebook in a position to succeed.
Cons
For me personally, the only down-side I see is there can be a lack of career growth/advancement opportunities. Some of that has to do with the relative "flatness" of the organization, but it's still a CON.