Pros
Great company with an amazing culture. You work hard, the pace is fast, expectations are high, and change is constant, but Facebook really looks after its employees and provides a diverse array of opportunities for self development, cross-functional training and career growth. The CEO meets with all employees once a week and answers anyone's questions.
Most people here are energetic and passionate about their work and seriously focused on making an impact. The work environment is physically (and culturally) very open: open spaces, no offices, casual dress, etc. Work/life balance is respected and encouraged and you have the flexibility to work in the way that best matches your individual style and needs. Face-to-face communication is highly valued but meetings are generally focused on not time-wasting activities. Anyone can suggest a new feature or improvement and be part of the team that makes it happen. There is a lot of opportunity for high performers to grow their careers. The compensation, benefits and perks are truly above par.
One of the most important things for me is the open culture and the way Facebook encourages employees to find and be our authentic selves, and to listen to the voices of others who may be different from ourselves. The company works very hard to create a diverse and welcoming environment for all.
Cons
It may be hard to adjust in the first few months if you're used to working in a traditional company. You can't sit in a corner and work alone - cross functional teamwork and building relationships are the norm. Facebook is always in the public spotlight, and this is one of the things you'll have to learn to deal with if you want to work here. And you'll have to get used to spending more time on Facebook. :-)