That's a tough one. We always knew that Sakuga City would work, at least as a small community since the core of it is 14 years old. YES, absolutely I still had to work to get it active and try and attract in new blood. I've made over 1,000 posts on the site myself and believe it or not, I wasn't over-inflating things, just posting genuine replies to topics.
You need a lot of time. I found that if I go inactive for any period of time, so does the forum because I'm the admin who's most visible. The others do post too when they have time, one of them is our coder, so he could be busy with backend... but it's scary how much activity can depend on even one person kicking life back in.
I've posted here for packages as well at times when it needed a kick up the butt. I don't think it's ever easy. ^^
If it hadn't worked, we'd probably have stopped since it does cost money to run. Xenforo license, add-ons just to get the place running smoothly, and we are outgrowing the damn server again I think. We are running it on money given by the membership though, so admins aren't paying for it.