The spirit (if not the letter) of the law that now hovers over political advertising is the equal-time provision of the Communications Act of 1934.
The equal-time rule doesn't apply to the internet, but the recent hubbub over Facebook’s influence on the election—in which both Hillary Clinton and Trump’s 2016 campaign digital director, Brad Parscale, chimed in—illustrates that many feel it maybe should.
The spat began with an explanatory piece about how Facebook’s ads auction, which determines who pays what to reach certain people, and rewards what the company politely terms “engagement” (and what we’d call clickbait).
The plots fluctuate wildly, but they show Trump generally paying a bit more than Clinton, which seemed to defuse Parscale’s tweeted claim that Trump massively underpaid on media, but doesn’t really answer much else.
What’s absolutely true is that the candidates were charged different prices for media, possibly even for the same media if both were contesting swing-state voters.
Starting in the now-ancient history of the commercial internet during the late '90s, online ads were sold much like the TV and radio ads that preceded them: a certain amount of real estate on a certain destination at a specified time, not very different than a Super Bowl ad.