I also have one more theory which is that this effect can be caused by the network aspect of Facebook. Maybe I haven't been interested in a specific topic, but one of my friends was. And Facebook is serving me ads because they were relevant for my friends so they might be relevant to me. And the conversations where I heard about a topic are a consequence of that same thing -- one of my friends is interested in a certain topic.

I don't know how valid it is though. But it could contribute to the whole thing I guess...