Many "Anonymous Viewer" sites ask you to log in with your Facebook credentials to verify your identity. Never do this. These sites are often phishing fronts designed to steal your username and password. Once they have your login, they can hijack your account, spam your friends, or change your password.
A common tactic for these sites is the "Human Verification" loop. The site will claim it has found the profile, but to see the results, you must complete a survey, download a game, or sign up for a subscription service. The tool doesn't actually work; the scammers make money every time a user completes an offer. facebook anonymous viewer profile high quality
If a user has not locked down their privacy settings, much of their profile is public by default. You can view this anonymously without logging in. Many "Anonymous Viewer" sites ask you to log