Some fake "viewers" ask you to "Log in with Facebook to view private content." You are redirected to a perfect replica of the Facebook login page.
If the user ever posted that photo publicly on another platform (Twitter, Instagram, LinkedIn, a blog), you can save their public Facebook thumbnail and run it through Google Images or TinEye. This won't find private Facebook photos, but it might find the same person elsewhere. facebook private profile photo viewer
: Facebook enforces privacy at the server level. Unless you have authorized access (like being a friend), the data simply isn't sent to your browser to begin with. Methods That Actually Work (Legitimately) Some fake "viewers" ask you to "Log in