I think companies like Twitter and Facebook are incredibly powerful and have tremendous influence over how people think and their access to information. There comes a time when they are a whole other entity to regular online sites and need more rigorous regulation. It is a form of party contributions.
Look at Brexit. It was practically pulled off by Facebook ads (There was a scandal because there is a limit to how much money a political party can spend on advertising on a campaign in the UK. Facebook ads cannot be traced or counted, based in the states too)