What is the expiation for someone who used the internet in fraudulent ways that caused financial loss to other users whom he does not know, or the number of cards he exploited?

For repentance to be complete, usurped rights must be returned to their owners. If the owner of the right is unknown, the usurper must give the money in charity on their behalf with the intention of guarantee. In this case, the questioner must estimate the price of the cards they used and give that amount in charity. Allah will accept their repentance if they strive diligently in their estimation and are keen to give in charity.