I was first called by a FB recruiter and we chatted about 15 min about my general background information. I then was followed up with a phone interview with a FB engineer. He seems to be an experienced engineer. He started with a common background question like challenge of current project. Then, he asked me to design the Facebook Credit system. I proceeded with talking about I would started with designing the backend of the system, for example a ER diagram that would outline the relationship between existing user information and the new information related to the FB Credit system. I was a little nervous at the beginning, but the interviewer jumped in and gave me some help. He then asked me how would the total credit points of a user be calculated based on my design. For that question, I answered how the data is delivered to user from end to end in a multi-tiered web application. Next is the programing question (write a function to convert an ASCII representation of a positive integer to it's numeric value). I had a subtle bug in my code, but in general I got it right. I coded in PHP, the interviewer is satisfied with it since FB uses lots of PHP. The last 5 min was for me to ask him some questions. The interviewer was very professional and helpful during the interview. He was able to follow my thoughts and provide hints.