It's easy if you have access to a big list of users from somewhere else.

I used to run a stack of private torrent trackers. The first one I made came from the fact that the admin of another site ran off with all the funds and abandoned the users. I moved them all to my site. Then every time I set up a new site I had thousands of users I could message to let them know. On the final one I had 100K+ active users - a message to them was like having your own private army to do your bidding for you.