But I just had an anecdote, and I am positive that google use info gleaned from private message to target an ad on youtube.
1) You talk to a friend, in-person or by message, about [topic]
2) You go about your business
3) Your friend Googles [topic]
4) Google associates [topic] with your friend's ad ID, as well as the ad IDs of anyone they've been near in the past 24 hours (you)
5) You and your friend see ads about [topic]
I don't know how smart they are about it, but it's likely working on the same system that runs their location services "Your reviews/photos are being read on Google, answer questions about [place you recently visited]?", and Google Rewards which tracks your location to ask you about receipt data and other things. It's very good at knowing if you've been into a lcoation and spent time there.
So, if they know a group of people were at a location for a period of time together, and those people have done so before, they can show ads based on one group member's search history to all of them.
No creepy message reading, or sneaky microphone recording, just regular creepy location tracking.