What you can do is, identify common problems and mods within the linux niche. Post threads that contains the solution to a problem but make it visible only to people who registers
Hi. There is really one question why you should really answer. Why on earth should they join your forum? Now I am not being bad but if you are answering their questions through articles then forums are then needless time consumers. But if you start to post advanced articles, your opinions, uptodate/ insider knowledge then that will generate curiosity. Also try to get an email subscriber base up and going so that they can then start using the forum and you can start using the forum to post to your readers. Also are there forums on your topic anyway and are people joining those? Can you interview or ask people in the related field to chat with you and then post that in your forum. A lot of forums work on that basis. Create a banner on your site that says "ask a question" which then links to your forum. Also make sure the forum has rules before you start attracting members and figure out a way to enforce those rules (through true moderators). Totally open forums tend to be free for alls. Forums like DP have an enticement model behind them- more you contribute good content the better your forum experience which gets a better forum which attracts better users etc- the system becomes cyclical. People in the niche will then contact other people (if the forum is supposed to do what it says it does) and the organic linking starts. But before you go big, just make sure that everything is properly laid out because in small niches it will be hard to build trust back up. Regards.
Here are best tools to get traffic to you forum: 1. forum posting 2. social media marketing (FB, twitter.....) 3. Blog posting 4. Search engine submission 5. Directory submission Thanks