HR at a large company is not like HR at a small company. Broad strokes have to be taken to make the process manageable at scale while being somewhat fair. Generally speaking, the phone screen comes after any pre-requisite criteria are checked (such as programming competence). Phone screens aren't very useful from an interviewing standpoint anyways; most simply exist to weed out candidates that are non-adjusted or have interests that lie entirely elsewhere (e.g., I only want to work here before I go back for my M.S. in CS in 6 months). They are more for the benefit of the interviewee if the company isn't a household brand; I think you probably already know most things you need to know about Facebook.