the 'fake posters' thing can also be duplicated by giving people important jobs like making them moderators of sections etc. Then they take pride in it and bring in their friends. If you have a few people with a few accounts each it doesn't take long for conversation to take off.
It also really depends what the subject of the forum is ... people might arrive there because they want to look at something on your site, but some subjects just won't encourage conversation no matter what you do.
hit me up if you want to link swap:
http://www.aussiecelebs.com