Based on what I have heard, FB is a very demanding workplace. Lots of smart people trying to get ahead and with that comes a cutthroat culture and poor work-life balance. As an engineer you will have access to smart peers, which can make a big difference to your technical skills. Pay is very good, as is the name-recognition, which can both make up for FB's negatives. If you are planning to start a family, look for a less demanding workplace; otherwise, you can't lose much by doing a 2-year stint there.