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Yes this definitely happens. I've had people on the same network as me getting Japanese YouTube ads just because I or someone else looked at a few Japanese videos.

I think this is the only model for Whatsapp. They know who is talking to who. They don't know what you are talking about but that doesn't matter when one or two persons in a group show some interest in a subject.

Load balancers call this IP Persistence. It's one way of keeping users connected to the application server that has their session in memory. Except in this case, the session is ad info about the user.