Big data advertisers that work with multiple different websites track users across thousands of sites that they work with and then piece together their entire portfolio of online activity and sell it to companies that want to figure out how to convince you to buy shit you don't need.
Just so everyone is aware of the capabilities here, I used to work for a nationwide self-storage company and if someone clicked on a Hulu ad to reserve a unit, we'd have their CV/resume, parsed through a web API by some company called "Sovren", and their whole profile fed through a machine learning model to determine how likely they were to actually move in and whether we needed to actually keep a unit empty until their supposed move-in date (because it was a relatively competitive industry, just empty rooms basically, there were no reservation fees).
Big data advertisers that work with multiple different websites track users across thousands of sites that they work with and then piece together their entire portfolio of online activity and sell it to companies that want to figure out how to convince you to buy shit you don't need.
Just so everyone is aware of the capabilities here, I used to work for a nationwide self-storage company and if someone clicked on a Hulu ad to reserve a unit, we'd have their CV/resume, parsed through a web API by some company called "Sovren", and their whole profile fed through a machine learning model to determine how likely they were to actually move in and whether we needed to actually keep a unit empty until their supposed move-in date (because it was a relatively competitive industry, just empty rooms basically, there were no reservation fees).