I wouldn't go as far as having fake users online. But I have set the duration of people appearing online to 60 minues in the forum settings, which is the maximum. I think a forum with many users appearing online will be more appealing, and people are more likely to become a member, because in a technical forum like mine, the biggest motivations to become a member are: getting answers to technical questions, and showing off projects that you have done. Both require an audience.