Even this commonly parroted "fact" isn't true. Cambridge Analytica exploited a Facebook misfeature that let them scrape a frankly ridiculous amount of data from users who took their quiz thing. Certainly Facebook shouldn't have allowed random apps access to this user data, but straight up selling data would be an act of immense incompetence on Facebook's part. Their whole business is keeping the data to themselves and selling access to targeted advertising based on that data.