[AJ007]

I'm curious about the Slack thing. I wonder if there could be third parties doing something (browser plugins, third party keyboards for Android, edit: someone using a TV as a computer monitor.)

One thing is for certain, if ad targeting is not being done in ways it shouldn't be, there isn't anything technically preventing it.

[TrainedMonkey]

It could be as simple as links. People drop links in the slack discussions, other people from Geolocated IP addresses (or same) click on them. Google analytics et. al. hovers a lot of data.