From my personal experience (I applied for a full-time position), Facebook interview process is very standard and is almost the same as other big companies like Google, Linkedin and so on.
Basically I had 3 phone screens at the beginning. One is with HR, which is very short and basically we just talked about what I did in the past and there's no behavioral question asked. So you definitely don't need to prepare anything for this except getting familiar with your past experiences. The rest two are all technical interviews that I was asked to write code on an online code sharing tool. I was asked 2 questions at each interview and the problem is not really hard like bit manipulation.