In a sometimes-heated hearing in Washington last April, 55 U.S. representatives questioned Facebook Inc. Chief Executive Mark Zuckerberg about privacy concerns and leaked user data. In the week before the U.S. midterm elections, about two-thirds of those same representatives are spending campaign dollars advertising on Facebook.

Politicians’ enthusiasm for targeting potential voters and donors on Facebook cuts across party lines—as did their criticisms. Paul Tonko, a Democrat, told Mr. Zuckerberg at the time, “Users trusted Facebook to prioritize user privacy and data security, and that trust has been shattered.” Republican Tim Walberg expressed concern that Facebook was banning political content and advertising based on the views expressed in it.

Campaigns for both have subsequently sunk money into Facebook advertising, according to a tool Facebook recently released that allows anyone to look up ads for political campaigns and “issues of national importance.” Neither congressman’s campaign replied to requests for comment.

Rep. Greg Walden (R., Ore.), who ran the hearing before the House Energy and Commerce Committee, has placed hundreds of ads on Facebook since April, when Facebook’s database of political advertising begins. “Greg Walden reaches voters in Oregon’s Second District across all mediums. That includes connecting to voters online, on social-media platforms, and via radio, television, and print newspapers,” says a spokesman for his campaign.

That few politicians feel they can escape the necessity of advertising on Facebook is precisely why we need to contemplate its ever-growing scale, revenue and power. The ramp-up in political spending across Facebook’s social networks, which also include Instagram, is breathtaking: In 2014, digital ad spending was 1% of all political ad spending. Now it’s 22%, or about $1.9 billion, according to the nonpartisan Center for Responsive Politics. Facebook says that politicians have spent nearly $300 million in the U.S. on Facebook ads since May. As of Oct. 30, Democrats were outspending Republicans on Facebook 3 to 1.