Moveon.org are looking for volunteers to help find and contact the companies whose ads are appearing on breitbart.com - see https://docs.google.com/document/d/1d3uLPk-TfwmpWUozHwR3jh--n3VV0e6odT-47sQhm8o/edit for details and how to participate. You have to turn off your ad blocker, obviously, and if you want they have a convenient browser extension for Firefox and for Chrome to help you out.
These advertisers are called "hidden" because by and large they likely do not know where they are advertising. Instead a third-party ad broker sells spaces that these companies purchase from the broker. They simply collect revenue without any awareness of the context in which their ads appear. So goes advertising on the Internet now. The question becomes "Will these companies care once told that their ads are appearing on breitbart.com?"
If you'd like to help find out, check the Google doc linked above.
Cross-posted from Dreamwidth, at http://drwex.dreamwidth.org/949378.html. Please comment there; this journal is winding down.