What is incumbent upon a penitent who has committed heinous sins against many people online, and finds it difficult to absolve himself of their grievances due to not remembering their identities or contact details?

Repentance from injustices related to people requires abstaining from them, regretting having committed them, and resolving not to repeat them. One must also seek forgiveness from the one who was wronged. If it is not possible to reach them, then one should pray for forgiveness for them and for oneself, for the door of repentance is open.