Pros
Having been here for a few months, there is an incredible level of freedom to contribute in many different ways. As a developer if something needs fixing, its gets tabled for fixing. I've been more used to hierarchical approaches to management, so facebook is a refreshing change, allowing me to bring forward many different aspects of my skill set, not just the narrow set defined by my job title. Diverse and welcoming company, offering many opportunities to learn new skills and projects.
Cons
Took a little while to get used to the lack of being told what to do. It may seem a bit chaotic at first, but ultimately that is an incredible freedom.