The grandparent doesn't say this specifically, but I've long suspected that a middleman adversary can identify, with high accuracy, which top-1000 website you a

The grandparent doesn't say this specifically, but I've long suspected that a middleman adversary can identify, with high accuracy, which top-1000 website you are accessing (even through a VPN) based on the timing and bandwidth signature of the encrypted traffic. Probably to a lot more detail if they gather and analyze longer-term traffic pattern data. They can certainly tell whether you're using streaming video with this type of analysis. If people start using VPNs en masse, ISPs and/or other interested actors will develop this technology, if they have not already. (I would guess it already has been, albeit perhaps not widely deployed.) If the ISP is the same as the ad network, or a data broker with one (and some are, see: Verizon Wireless), they can then link your IP address to a cookie-based profile. (Yes, there are counter-measures there, like disabling third-party cookies. And there are countermeasures to that, like browser fingerprinting.)