I think paid content versus unpaid content is a useful distinction. I think we also need to remember though that the content is important in deciding whether this is political or not. It's the dissemination of the content that makes it the advertisement. You could pay an artist to create a political campaign message that you leave in your house. That's paid and it's political but it's not being disseminated to others. The distinction there is important because things that help disseminate content that was not paid, I think, could reasonably still be considered an advertisement.
Imagine somebody writes a post on Twitter, they themselves don't pay to have it promoted, but somebody else chooses to promote it—