@ACE
We can't help you until you reveal more details about your forum. Advertising is different always, for example as others said warez forums get more users if they hide the links to guests/search engines. While porn forums get more users if they force users to register after they read more than 10 threads. If you have just an average general discussion forum, it'll be kinda hard. But adding more sections for discussing about current shows on tv, music, video games, and advertising it via Facebook would work.
If your Facebook plan works, your users will talk about your site in conversations with their friends, which would automatically complete the "get users" part.