This is a scam. DON’T tap on the button, as this can escalate the situation. It is likely you are just being redirected to a website impersonating Facebook. To make sure, tap on the address bar to see if it doesn’t use HTTPS or the link looks sketchy. If this is the case, you can simply press the tab manager and close the tab. I would also suggest clearing browser data in case. But since you said it was persistent, it could be possible you have adware on your device generating pop ups everywhere you go. You can try to:
-Clear browsing data like I said before to remove cookies and cache that may be causing the pop up
-Delete suspicious applications you never installed
Reasons why the pop up is fake
-It uses urgent language which is a common tactic used by scammers
-It states you visited an adult website, which also, is another source commonly used by scammers
ALWAYS be aware that these pop ups are scams. Whatever they say is a lie and should NOT be trusted.
You can also reach out to apple support if it gets worse.