Twitter CEO Jack Dorsey just took a decisive stand in the ongoing public debate about political advertising and messages on social media. He announced that, beginning November 22, Twitter will not run political ads at all.

The announcement comes less than a week after Facebook CEO Mark Zuckerberg faced rigorous, and sometimes downright hostile, questions from members of Congress about whether the company should fact-check political advertising. The controversy arose after the Donald Trump presidential campaign ran an ad on Facebook accusing Joseph Biden of corruption, even though CNN and others refused to run the ad because it did not meet their standards for truthfulness. Presidential candidate Elizabeth Warren then upped the ante by running a fake ad on Facebook saying that Facebook had endorsed Trump.

Dorsey, observing all this, no doubt concluded that he could save himself and his company a world of grief by pulling the plug on political advertising before the 2020 election cycle gets fully under way. But there were principles at stake in the decision as well, as he explained in a series of replies to his own original tweet announcing the decision. For example:

As nearly everyone has pointed out, this is a relatively easy decision for Twitter, since political campaigns spend much more to advertise on Facebook and elsewhere than they do there. Twitter's CFO tweeted that political ads brought in less than $3 million of Twitter's revenue during 2018 when midterm elections took place. That's one-thousandth of the company's typical annual revenue of $3 billion.

It's also the right decision. Some have questioned the absolutism of banning political ads altogether, pointing out that television stations lack this option and are required by federal law to run them. But there's a fundamental difference between advertising on social media and advertising anywhere else. Unique among the companies we interact with on a daily basis, social media companies are able to collect and record detailed, granular information about our likes and dislikes, what interests and bores us, who friends are, and our political leanings. In fact, Facebook will put you into a political category such as "liberal" or "conservative" even if you're smart enough to never discuss politics on Facebook.

Facebook uses the information about your demographics and interests it's able to glean from your posts and who your friends are, and uses it to micro-target ads just to you and people very much like you. This means that political ads with very slanted or even false messages can still be very effective, because they can be aimed at an audience known to be very receptive to whatever message they convey. (This is also true of all other advertising on Facebook, a useful thing to keep in mind when you're on the platform.)

The potential for harm from this type of advertising is limitless. For example, in 2016, ads bought by Russians on Facebook encouraged voters to protest by staying home on election day or by voting for the Green Party candidate--and were disproportionately targeted at African Americans. As a result of that coming to light, Facebook now has a policy to ban ads that discourage voting. But it's still open to all kinds of other mischief.

Dorsey still has some details to explain. For instance, he tweeted that the platform would ban not only ads for candidates but also "issue ads." As he points out, allowing issue ads would provide a way for politicians to circumvent the ban on political advertising. Besides, in effect, it would mean that everyone but political candidates could advertise on the platform, which doesn't seem very fair. But how, exactly, will Twitter define an issues ad? Dorsey hasn't said, but he promised to provide all the answers on November 15. Until then, we'll just have to speculate.