people whose profiles are public, even if the target's own profile is locked.

If you encounter one

: This is the only official way to gain access to a user's private content with their permission.

Thus, a working, free, online private profile viewer is for any website, including iStaunch.

Short conclusion

If you have already entered your credentials into a fake viewer, change your Facebook password immediately, check for unrecognized login sessions, and enable 2FA.

If you need to see a private profile, the only reliable and safe methods are: Control who can see what's on your Facebook profile