Quick sidebar: hiring is super important for us at Facebook. We want to keep working with the smartest people on the planet, who also fit in culturally, and so interviewing is one of the things we take really seriously.
The one thing that we are also very serious about at Facebook is that once you get the job, we want you to be doing that which you are most passionate about, because we know that when you are doing something that you are passionate about, you will produce your best work, and be happy doing it. We don't just slot people into where we think they fit, or where we have a gap, simply because the skills on their resumé fit. A lot of time and effort goes into introducing new hires to as broad a swathe of engineering teams and activities as possible to allow them to make an informed decision about which team they want to join.
We also don't tie engineers to any one team for the duration of their tenure at Facebook, and actively encourage them to not get in a rut, regularly move teams and challenge themselves with something they're unfamiliar with. Personal success and career satisfaction is very much up to the individual in many respects. And this applies equally to individual contributors and managers.
What I love most about working at Facebook though is still being able to have a tangible impact. I have seen it grow from < 100M users to > 1B users, and seen the infrastructure and engineering teams grow with it, and all without negatively impacting the scope to actually do stuff. And I don't just mean small iterations on a big bit of software. We constantly evaluate our tools and systems, and are not scared to discard what no longer works, and regularly build new and highly impactful stuff from the ground up. Oh, and there is seldom any precedent to go by either!
It is a fantastic place to work, with lots of great people, who are all insanely smart and very motivated.