Many "private viewer" websites are designed to steal your Facebook login credentials, personal information, or even financial details.
If you are in a situation where you feel unsafe due to a private profile (e.g., in a case of stalking or harassment), you should report the account directly to Facebook rather than using third-party tools. Proactive Steps for Protection If you're concerned about your own digital footprint, How to report a fake profile that is using your photo? facebook private profile picture viewer online