The developer reported a bug in Facebook on the page Zuckerberg

If your Facebook page is not public, then others, in theory, are not able to write anything on it. However, the developer from Palestine, Khalil Shirteh, discovered a vulnerability that allowed any user to place a link on another page, writes The Verge.

Shirtech reported an error on Facebook hoping to receive a reward, but the company ignored the problem, replying that it was not a bug. As a result, using the same bug, he posted a message about him directly to the Mark Zuckerberg page.

Before reporting a bug on Facebook, Shirtech tested it on Sarah Goodin, Zuckerberg's girlfriend. In a letter to Facebook, he described the details, noting that the security service might not see his post on the Goodin page, since her profile is open only to friends. Despite the post screenshot attached, a Facebook engineer who called himself Emrakul replied: “I’m sorry, but this is not a mistake.”
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Unsatisfied with the answer, Shirtech decided to notify Mark Zuckerberg about the bug by posting a post on his page. A few minutes later, Facebook engineer Ola Okelola contacted Shrittech, requesting detailed information about the bug. Facebook has disabled the Sritech account, apparently fearing other consequences.

Now, Sritech’s account is activated again, but the company claims that “there was not enough technical information” in its initial report about the bug. In a letter to Shritech, a Facebook engineer who introduced himself as Joshua wrote that the company "cannot pay you for this vulnerability because your actions violated our terms of use for the service."