Most of these websites operate on a "human verification" model. They will promise to show you the visitor list, but first, they ask you to log in with your Facebook credentials or complete a survey.
Would you like a shorter version (e.g., for social media or a warning post) or a technical breakdown of how these scams work under the hood? facebook locked profile picture viewer online better
If you prefer not to use third-party software, there are technical methods often cited in community forums: Most of these websites operate on a "human