There's a widely held belief that Facebook and Instagram listen in on users through smartphones, and then serve advertisements based on what was said aloud.

Facebook and Instagram have repeatedly denied doing as much.

Despite those denials, and evidence from experts to the contrary, the belief is widely held and seemingly unshakable.

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There you were, talking to Gina about a potential trip this fall to Buenos Aires. "Maybe I'll go to Buenos Aires this fall!" you said. "Or maybe Lisbon! Who knows!"

Hours later, idly scrolling through Facebook and — what's that? An ad for vacationing in Lisbon? How could it have known?

It's perhaps too familiar of a story at this point. A cliché. "Facebook is listening to my conversations!" you tell Gina. "I know it." And she's sympathetic. Maybe she even has her own story about something very similar happening to her.

Look no further than CBS This Morning anchor Gayle King, who related a similar story to Instagram head Adam Mosseri in an interview in June.

"Can you help me understand how I can be having a private conversation with someone about something I'm interested in seeing or buying, and an advertisement for that will pop up on my Instagram feed?" King asked. Even after he explained how it could be happening, King wasn't convinced. "I don't believe you!" she said. "I don't know how this happens repeatedly.

The belief that Facebook and Instagram are listening to users through their smartphones, then serving ads based on that spying, is extremely pernicious.

It crosses generations, race, gender, and income brackets. Your conspiracy-minded uncle and members of Congress and your favorite morning news anchor are on the same page for this one. Everyone, it seems, believes that Facebook and Instagram are listening in on them.

And no matter how hard they try to tamp down that belief, it persists.