You probably saw it dozens, if not hundreds of times in the months leading up to the US presidential election last year.
A political ad would air. And at the end, you'd see a clear message about who paid for or endorsed the spot, whether it was Donald Trump, Hillary Clinton or any of the numerous candidates or political action committees.
Such messages have been the standard in political advertising since 2002, when Congress passed a campaign reform law that required them. Now, whenever a political message runs on television or in any other traditional mass media, you can see who paid for the ad. I think we can all agree that kind of transparency is a good thing.
However, that disclosure requirement doesn't apply to online ads. As a result, when you see a political ad on Facebook or Google or other sites, it can be impossible to tell who was behind it, whether it was a particular candidate, a PAC, a non-profit — or a fake account run by groups linked to Russia.
You just don't know.