OK, this one hasn't been proven, but it's the conspiracy theory that won't die.
There are countless accounts of Facebook users who received targeted ads about a subject they had just had a face-to-face conversation about, even if they had never posted about the topic or searched for it online.
The stories often sound a little too suspicious to be mere coincidence, but Facebook has persistently denied that it is listening in on its users.
Meanwhile, Apple, Google, Amazon and Microsoft have faced accusations that their voice-activated personal assistants are listening on people's conversations when they aren't activated by voice commands.
The companies have denied the rumors, however, and have pointed out that while their tech does record commands given after they've been activated, users are welcome to go online and delete them.