In this age of rage, Madison Avenue is finding itself on red alert.

Advertisers are increasingly in the cross hairs of populist activists — aided by the power and reach of social media — who are demanding that brands quickly take sides on divisive social and political issues, posing a new challenge to corporations that usually prefer to stay out of the fray.

After a groundswell of online anger over reports that Bill O’Reilly, the Fox News host, had settled with at least five women who accused him of harassment, more than 50 companies pulled their ads from Mr. O’Reilly’s popular prime time program. The exodus followed similar campaigns to pressure brands with ties to President Trump, like L.L. Bean, Uber and advertisers on “The New Celebrity Apprentice.”

“Americans are now demanding that their brands articulate their values and weigh in on political issues, and I think the degree to which they are expecting that is really quite new,” said Kara Alaimo, who teaches public relations at Hofstra University and worked in communications for the United Nations, the Treasury Department in the Obama administration, and the administration of former Mayor Michael R. Bloomberg. “What social media is doing is forcing companies to make these decisions much more rapidly.”

Ad boycotts are not new: provocateurs like Don Imus and Glenn Beck lost their cable news soapboxes in part because an angry public used petitions and letter-writing campaigns to force companies to drop their sponsorship.