My school sent out an e-mail about the jobs available at Facebook and a link to the school's job database site. We were required to answer a few short essay questions like explaining how to use certain features on Facebook and what we would change, along with our resume. (It was to test our writing skills.)
Less than a week later, I got a call from my school's career center saying I needed to schedule for an interview in a couple of days.
At the interview, I was incredibly nervous. It was my first interview, and I had no idea what the job entailed. The job description was vague.
The interviewer was changed at the last minute. HR was supposed interview me, but the manager of platform operations (if memory serves me right) decided to interview everyone that day, so that made me even more nervous. She asked me probably 25-30 questions, and I felt like we were having a staring contest. After every answer I gave, she would stare at me stone-faced for several seconds until I said,"That's it," before typing my answer down in her notes. Very awkward experience.
By the end of the interview, I realized I was (unfortunately) overqualified for the job. It also turned out to be a non-technical position with no room to move up. We ended things on a good note once I started asking her questions about her experience at Facebook. She warmed up quite a bit.
One thing I can tell people to make sure they do is: always compliment or have positive things to say about Facebook. Obvious advice, but they really like it. Oh, and don't mention Google or any other company at all, unless they ask who their rivals are. Even mentioning "'googling' something" seemed to make my interviewer cringe. (I made some rookie errors, since it was my first interview.)
I'd say the difficulty of the questions was about average, but the non-stop staring made the interview even harder as I had to maintain constant eye contact.