Facebook is the worst job, except for all the others
Pros
Facebook is probably the best big company to work for. The company cares a lot about having its employees being productive and satisfied with their jobs and it works hard at maintaining an environment to support that. As a survivor from the real world, this is really rare -- pretty much every company-wide system is designed to make life easier for employees. The benefits and conveniences offered are amazing. Many decisions about the company and its goals are made fairly transparently and open discussion about most things is encouraged. This can get really annoying at times. Unlike most companies, Zuck is actually a pretty inspiring leader. He makes mistakes, but does learn from them. He has really lofty goals, though, and doing the right thing is highly subjective when trying to change the world. If you are politically savvy and willing to work hard and make sure others notice that hard work, you will go far. You will learn a lot if you seek out opportunities to learn, and work with a lot of very talented people. You have to own this yourself, though; don't count on a lot of support.
Cons
Facebook still believes that it is a magical meritocracy reasonably free from corporate nonsense. While there aren't any Office Space-style TPS reports to fill out and you don't have to bring in your own coffee and toilet paper, it is now just a big company job like any other. If you want something, whether it be a promotion or a good dessert from the cafeteria, you have to plan how to get it, execute on the plan, and then make sure everyone else knows what you did and how great it was. You also have to share just enough detail to be convincing but not so much that people can poke holes in your story or usurp your dessert acquisition techniques. The typical first level line manager at Facebook doesn't have prior management experience, only knows the Facebook way of doing things, and is overwhelmed with all the pressure being placed on them. It's a really hard job, too; your manager will be responsible both for setting the direction of your team and for making sure you and your colleagues grow at the company. Most people are not good at either of those things, let alone both. Be aware of what your manager is looking for and how to best communicate it. Everything FB does is subject to a lot of public scrutiny, and you will know how the sausage is made and see that the goals of the company aren't focused on invading privacy or global domination like everyone else thinks. It's hard to see this thing that takes up so much of your life lionized and loathed by the press and your friends. For engineers, a lot of the tools you will use day-to-day are incomplete, broken, poorly documented, and might not be supported anymore. You will get used to this. "Move fast and break things" is more complicated than it sounds, and that's really hard to adapt to when coming from a more traditional company. Have big goals but chop them up into small tasks with well-defined success indicators and communicate them early and often.
