Facebook gave (gives?) feedback. It's good PR in the candidate community, not just bet candidates like it, but because it promotes Facebook's reputation as a fearless company that does what they believe in regardless of conventional wisdom.

Uh no they don't. At least not in all cases. They refused to give me feedback after requesting it. I even asked an inside friend to follow up with the recruiter.

There's a perceptional upside. It's difficult to measure, however.