As a Facebook engineer, you can have as much impact as you are driven to have; your opinion matters a lot!
Pros
As a software engineer, a lot of work is self driven and based on self-motivation. You are not told exactly what to do week over week. It is your job to figure out what is best to work on, what has the most impact for the company, and how to improve things. Most engineering is driven by the engineers themselves and not some sort of management. To me, this is perfect. However, if you don't like thinking about what you are doing and being critical about what is good for the company and your team, this might not be the place for you.
Cons
Facebook is all about iterating fast. This is great for our products because we can quickly get something out there and see what works and what does not. However, it means we have a lot of sketchy software and products sometimes. It also means we have to constantly be doing "cleanup" which often takes a long time. You will be under "attack" by the outside world because things "suck" even though you know they are being worked on and improved, but that takes time.
