Comment by vezzy-fnord

https://www.facebook.com/whitehat

      Exclusions
      The following bugs are not eligible for a bounty (and we do not recommend testing for these):
      ...
      Denial of Service Vulnerabilities
      ...

That said, they do appear to be quite hypocritical when it comes to enforcing this, as at least one user on the /r/netsec thread claimed that Facebook paid them for a mail bombing vulnerability.

I guess you have to use this to attack high-profile targets (or Facebook themselves) until they notice.