There is a requirement to set people on the right path when you see them straying. This is called giving Tochacha. What are the requirements for giving Tochacha? Are there any limitations on who can give it or to whom it may be given? What is the current suggested approach?
According to Rambam:
If what one is correcting his friend for is a transgression between Man and his fellow, then one may not use any type of tone, language, name or label, or even content that is embarrassing to the correctee. If one does then he has transgressed the very grave sin of embarrassing his fellow in public about which our sages of blessed memory said results in losing his portion in the world to come. Rambam learns that this general prohibition to publically humiliate another is actually learned out from the mitzvah of Tochacha.
If what you are correcting him for is a transgression between him and Heaven, then if after you correct him in private he still does not correct his ways, it is permitted to even publicly humiliate him until you get him to change his ways.
A few notes:
It is clear from Rambam that the corrector must know the correctee.
The measure of what is considered public is based on potential humiliation by others hearing. This would include an internet setting.
Even those sins in which it is permitted (after attempting in private) to humiliate, this is only if this will motivate the person to change. Which leads us back to my first point.
This mitzvah is only a mitzvah if the correctee has really done something or acted in a way that is sinful. If it is a matter of doubt or subtle judgment as to whether the action or behavior is against the Torah, then a reliable Poseik must be consulted before attempting to correct. One's own personal involvement or subconscious grudges can be very blinding in these matters.