Like working at a start-up, a very big one. The friendliest and fastest work environment I have ever been part of!
Pros
Working at Facebook has been and still is the most amazing experience, for me. Why? Because, - The people that work here are extremely bright and driven - The work environment is incredibly open and supportive. If you want to get something done but need help or input from a different team, you will get it - You have the freedom to focus on what you feel will have the highest impact. Of course, you still need to do you core job though - The work culture is truly flexible These things may not be the most important points for you but after having worked in the voluntary sector (NPO), public sector (Army and UN), and the private sector (Banking) they are the most important to me. And from my experience separates Facebook from 99% of all work places.
Cons
If your require a lot of prescriptive guidance (step by step instructions), don't like dynamic work environments, and are lethargic when it comes to your work, this is not the best place to work at.