You are interpreting this to mean "positions", but "offers" normally means a specific person was told "congratulations: we've agreed to hire you! here is what we will pay... are you in?" and so it does seem very strange that half of such offers are being rejected (and then implies the thought process explained of maybe people interviewing for multiple jobs and half the time choosing someone other than Facebook). (I honestly don't know which is "correct" for what is actually happening, but I noticed the disconnect between your two comments.)