Facebook's response is insightful. They only run ads through their own network.

  # Facebook.com/ads.txt 
  #
  # Facebook inventory can only be purchased from Facebook advertising 
  # systems, and cannot be purchased programmatically through open 
  # exchanges. Therefore there are no authorized sellers or resellers 
  # listed in this facebook.com/ads.txt file.

https://www.facebook.com/ads.txt