Do not build or buy a "private profile viewer." If you see one advertised, it is either malware, a survey scam, or a phishing site. Use the OSINT aggregation features described in Section 2 only for ethical research on your own data or public figures.
A more sophisticated attack. The scam site asks you to paste a snippet of JavaScript into your browser's console while logged into Facebook. This code extracts your session cookies (tokens) and sends them to the attacker. They can then hijack your active session without ever needing your password. fb private profile viewer
Here are some common methods used by FB private profile viewers: Do not build or buy a "private profile viewer