It has been a while since I was at Facebook, so my information may be out of date.

• When I interviewed with Facebook, some of the interviewers told me that Facebook emphasizes hiring people who can code, whereas Google is more likely to hire ivory tower types who are good at algorithms. As far as I know, this is simply not true. Both Google and Facebook only hire people who clearly demonstrate good coding skills during the interview. The types of questions they ask in interviews are not that different.
• Facebook appears to fire people for underperformance more often than Google.
• Facebook is a “move fast and break things” company. You write your PHP code (I guess now it’s Hack, not PHP), test it in your sandbox, and push it to production the next week. Google seems to have much more stringent requirements for code quality, and releases new features much more slowly. Google has to have more red tape since it’s a bigger company.
• Facebook engineering is very focused on Facebook’s main product, which is the Facebook website. Google has a large variety of different products, but not all of them are considered important or prestigious within the company. If you work on something non-core, there is a high probability of your project simply being cancelled after a few years. (But don’t worry—Google very rarely lays off full-time Google employees. In fact I’m not sure whether it has ever happened.)
• Google appears to have better work/life balance in engineering, but Facebook engineers seem to make more friends at work.
• Facebook is more kool-aidy, so to speak. Its hiring process places non-negligible weight on culture fit, which, in their case, means that you should really love Facebook and believe that Facebook is changing the world. At Google, you are not expected to drink the kool-aid, and many employees, most of them engineers, are deeply cynical about the company.