50m Facebook User Accounts Breached

Discovered on September 25, 2018 - Facebook Engineers found 3 programming bugs (and they were breached) in their code that would allow hackers to take full control over the Facebook account using this feature to "view as".  The "view as" feature allows users to see what their profile looks like as someone else and was introduced July of 2017.  This security hole had to do with the ability to stay logged into multiple browsing sessions.

Facebook—"Can I know who's viewing my profile or how often it's being viewed?"

No, Facebook does not provide the ability to track who is viewing your profile, or parts of your pro file, such as your photos. Applications by outside developers cannot provide this functionality, either. Applications that claim to give you this ability will be removed from Facebook for violating policy.