What is the ruling on impersonating a girl online to talk to young men and convince them to repent from illicit relationships?

Advice is one of the greatest acts of proximity to Allah, and the Prophet, peace and blessings be upon him, made the entire religion about advice. What you did, lying and seeking out people's faults, is impermissible. Therefore, you must avoid such conversations, as there are many fields and noble means of da’wah (inviting to Islam).