Meanwhile Facebook is now refusing to put offers in writing without a verbal acceptance.
Human resources personnel and interviewers are likely frustrated with the experience of interviewing and arranging for offers for many people who are not accepting; this may be causing an appreciable decline in employee morale. Additionally, interviewing is fairly expensive, especially when the success rate (of hiring candidates that make it through multiple stages of interview) declines because of the effect I am describing.
Executives may have decided upon this course of action to correct the two problems I describe immediately above.