What is the ruling on money that its owner could not return to its rightful owners due to the impossibility of reaching them, especially since it resulted from a transaction with some doubt, and is it permissible to donate it on their behalf?

May Allah reward you for your inclination towards what is lawful and your fear of what is unlawful. What you did is safer for your religion and more absolving for your conscience, due to the widespread corruption and its dominance among those who use chat rooms, and the inability to control it. One cannot be sure that they will not be assisting sinners in their sins. Allah Almighty said: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression."

As for the remaining rights of the subscribers, if you can deliver them to them through their addresses, then you must do so. If you despair of doing that, then give charity on their behalf to the poor and the needy. As Sheikh al-Islam Ibn Taymiyyah mentioned, whoever possesses usurped items, or borrowed items, or deposits, or pawns, and has despaired of knowing their owners, should give them in charity on their behalf or spend them for the benefit of the Muslims.