"Micro-targeted" political ads refers to ads that are specific to a very small group of users. Using the same methods that target regular ads to users based on their search history -- such as how you may receive advertising about boating supplies if your browsing history includes articles about boating or visits to boating-supply retailers -- politicians can target individual internet users with ads specific to them and their interests.
For example, if an internet user visited hunting sites and purchased hunting paraphernalia, a pro-gun politician could target an ad to that individual touting the politician's stance on gun rights. In the same way, a pro-LGBTQ politician could target ads at individuals who, based on their search history, are LGBTQ or pro-LGBTQ.
In a letter to Facebook founder Mark Zuckerberg, hundreds of employees wrote that allowing these ads effectively limits the user to seeing only a part of a politician's platform.
"The risk with allowing this is that it's hard for people in the electorate to participate in the 'public scrutiny' that we're saying comes along with political speech," the employees wrote.
Or, as explained in NBC News, the ads aren't seen by a large-enough audience that they can be suitably fact-checked.
The employees also noted that Facebook has banned similar targeted advertising before, such as for housing, due to concerns about discrimination.
Zuckerberg, for his part, appears to have gotten the message: he's announced that he's "considering" limiting political candidates' ability to target specific users with political ads. Google later announced that is considering taking similar steps.
In banning micro-targeted political ads, Facebook strikes something of a compromise. The social media giant has been facing pressure to copy Twitter's lead and ban all political advertising, something Zuckerberg steadfastly refuses to do.
"Political ads are an important part of voice, especially for local candidates, up-and-coming challengers, and advocacy groups that may not get much media attention otherwise," he says.
Google, similarly, is looking at banning the micro-targeted political ads, according to a Wall Street Journal report. The tech giant has been holding internal meetings about changes to its advertising policies, and may announce changes this week. However, what those changes may be, and when they'll be implemented, remains unclear.
Twitter, for its part, has banned all political advertising, saying that the reach of a political message should "earned, not bought."