Facebook puts out stats from their bug bounty program once a year. Most of bugs are invalid reports - in 2013 they had 14,763 reports, with 687 being valid.
(https://www.fb.com/818902394790655)
They probably got a couple people working exclusively on bug bounty reports. I also have to say they did a great job changing communication channels from emails to tickets which show in /support/, it is way easier now. The downside is that you must have a Facebook account, not sure if it was needed before the change.