I dont think it's necessarily an ad. Just listed as a "related post" assuming it relates to something you posted. It would mean the Advertiser would need to create the post and you as a user would have the option to click on the post as if it was a post from any other user - and it would only show up if it was related to the topic of the post you are reading.

An example. You are reading a post from a user that is related to Bitcoin. At the bottom of the post would be a list of posts that relates to Bitcoin including any advertiser posts that relate to Bitcoin.

That is my assumption