There are a lot of reasons that political campaigns might prefer to advertise on networks belonging to Facebook or Google than on Twitter. Both Facebook and Google have broader reach than Twitter and are used by more Americans. Twitter users tend to be younger and wealthier than Americans overall, but that’s not necessarily who political advertisers are hoping to reach. Facebook has both a sophisticated tool set for campaigns and an older audience, for example, both of which make it more attractive to advertisers. Google, meanwhile, has a sophisticated advertising network that is pervasive on the Web, offering its own advantages.

This isn’t theoretical. Data on campaign spending from the Federal Election Commission shows how much money is spent on each of the platforms. Google and Facebook easily outpace Twitter. In recent cycles, Facebook has surpassed Google, in part on the strength of its campaign-focused tools. (We included Snapchat out of curiosity; it’s a distant fourth.)

(Those numbers exclude advertising that might have been purchased through contractors, such as political consulting firms.)

The difference is even more dramatic if you look at cumulative spending since the dawn of the social media era, the 2008 election. Campaigns have spent $46 million on Facebook (and Instagram) and $30 million on Google (and AdSense). They’ve spent less than $2 million on Twitter.

There have been about 64,000 Facebook transactions by campaigns and 34,000 Google ones. Twitter comes in at 2,600.

That’s important context for considering Twitter’s announcement Wednesday. Jack Dorsey, the company’s CEO, used his platform to tell the world that political ads would no longer be welcome on Twitter.

“We’ve made the decision to stop all political advertising on Twitter globally. We believe political message reach should be earned, not bought. Why? A few reasons,” he wrote. Among them? “While internet advertising is incredibly powerful and very effective for commercial advertisers, that power brings significant risks to politics, where it can be used to influence votes to affect the lives of millions.”

He took a moment to troll Facebook, which in recent days has been criticized for allowing false statements in political ads.

The announcement was not made without self-awareness, with Dorsey adding that he was “well aware we’re a small part of a much larger political advertising ecosystem.” A final policy, he said, would come in the middle of next month.

What will be more interesting to watch once that policy is released is how Twitter will handle issue advertising. Issue ads have been a loophole in political spending for years, with political groups often forming tax-exempt 501(c)(3) and 501(c)(4) groups to avoid campaign contribution and spending limits. There are rules guiding that spending, but they are lax enough to make issues advertising both common and contentious.

Dorsey addressed those ads, too.

“We considered stopping only candidate ads, but issue ads present a way to circumvent,” he wrote. “Additionally, it isn’t fair for everyone but candidates to buy ads for issues they want to push. So we’re stopping these too.”

This is a tricky position to take. Figuring out when a campaign is spending money is relatively straightforward (though not always trivial). Figuring out what constitutes an inappropriate political issue campaign is much trickier.

An ad from House candidate Joe Smith would be banned. An ad from Citizens for Green Space that touts Smith’s record on creating Municipal Park is a bit murkier but still pretty clear. But what about an ad from the Westside Neighborhood Association inviting people to come out to an event at beautiful Municipal Park, which is a central part of Smith’s campaign pitch? The line gets quite blurry, quite fast.

It’s the sort of can of worms that makes social media platforms wary of new efforts to police content. Now Twitter and Dorsey have cracked the can’s lid — and we’ll have to see how far back they’ll peel it, if you’ll forgive the extended metaphor.

For now, though, Twitter can enjoy one (obviously intended) effect of its announcement: digging at and emphasizing Facebook’s travails while giving up a relatively small piece of its own business.