If this hasn’t happened to you, then you’ve probably had a friend tell you this has happened to them. They were at a party, where they talked about something random like tennis rackets, and then the next day, when they got online, they were shown ads for tennis rackets. How did that happen?
In my personal experience, a lot of people assume that since they never typed the words “tennis racket” into their phone or computer, then somehow the companies that spy on us to sell us stuff must have overheard us speaking. They must be able to turn on our phone’s microphones to listen in on our conversations. It’s not a crazy idea, since devices like Google Home and Amazon’s Alexa exist, and they are able to parse what we say for the purpose of buying things and marketing to us. And anyone who is even vaguely aware of the claims of Edward Snowden will know that governments and corporations can not be trusted to only be listening when they say they’re listening.
On the other hand, with even a little technical knowledge, you’ll know that it’s one thing to get Google Home to understand what you’re saying when you’re speaking directly to it with a clear set of predefined phrases, and another thing for a computer to listen from a microphone in your pocket in a noisy bar and pick out relevant key words. And it’s a whole other level of computing technology to be able to take in what would presumably be hundreds of thousands of those unclear conversations from people all over the world and mine them for relevant data. While computing power is always getting more impressive, it still exists in a real world of mathematical limitations, and the amount of work might be worth it if you’re trying to stop a terrorist attack, but probably not worth it to just guess if some guy might want a tennis racket.
I believe my own variant of Occam’s Razor, which is that the truth is often the most disappointingly mundane option. In this case, the mystery of how it is you got an ad for tennis rackets after a conversation with your friends even though you never searched for rackets online has a simple answer. One of your friends did the search. Companies like Facebook and Google keep track of your friend connections and your contact list and your email and text conversations. They have an imperfect idea of which friends you are closer to than not, but it’s often good enough…