My first hack was a bulletin board I wrote in 1984 for a Commodore Vic-20. It had multiple message areas ("rooms"), private mail and an online game. Users could start their own rooms and make them public or private. There were about sixty rooms with topics ranging from string theory to general chit chat.
It was very popular even though it had only one dialup connection at 300 baud. The average user spent more than an hour on it.