Mark Zuckerberg is still explaining to the world how Facebook works

In a Wall Street Journal op-ed on Thursday, the Facebook co-founder once again stressed that his company does not sell user data. Facebook's business model is about selling ads, which allow the company to "offer services for free," Zuckerberg writes. He adds that selling user data to third parties would actually "reduce the unique value of our service to advertisers." While you can opt out of letting Facebook share certain information with third parties, he adds, you cannot stop Facebook from collecting your data

"We give people complete control over whether we use this information for ads," Zuckerberg writes in the piece, "but we don't let them control how we use it for security or operating our services."

When Zuckerberg says "operating our services," he also means running his business. That includes tweaking algorithms to make sure the posts at the top of your feed are from the people you communicate with the most and keeping track of your location to flag suspicious activity.

Facebook's data sharing practices have been under the microscope since it was discovered Cambridge Analytica harvested data from 87 million Facebook users, for the most part without their knowledge. To be fair, Zuckerberg did admit that Facebook's business model had put the company in uncharted territory in at least one way. "There's the important question of whether the advertising model encourages companies like ours to use and store more information than we otherwise would." Zuckerberg's op-ed did not provide an answer.