I have a pretty broad POV here, as I own a few forums and manage or work for a few others. IMO slots seem to be the slowest to get going.
In my own exp. I’m a lurker, I don’t even generally register unless I’m asked to, or I MUST reply to something. In that respect, controversy brings posts.
Aside from that there are a few ways to get forums going, there are programs like http://www.postonmyforum.com (the site has been sold, so I’m not endorsing, just mentioning, haven’t worked with them sense the sale) where you can pay for posts, or there are post trade programs where webmasters take turns posting on each others forums, because face it, noones going to join if there’s nothing to talk about.
You can also join under a few usernames and talk to yourself, as long as you do it right and it doesn’t look fake, no one joins a forum to talk to themselves, so you’ve got to build a starting point 
~LadyH