What should a person do who regrets their abuse and injustice towards others online, actions that have caused them obsessive thoughts and distress? Should they give money in charity on their behalf, or ask others to apologize on their behalf, especially if some of them are non-Muslims, and there are technical difficulties in communicating with them?

If you realize that you have wronged someone, then repent to Allah Almighty by abandoning the sin, regretting it, and resolving not to return to it. If you know the wronged person, then seek their forgiveness for your injustice. If you cannot reach them, then supplicate for them and seek forgiveness for them, because this is all you are capable of. By doing so, your repentance will be valid, God willing, along with increasing good deeds. As for giving charity on behalf of these individuals, it is not obligatory for you.