First of all, the lag when you log in pushes many players away, who assume it's there all the time.
Secondly, we must stick with the new players more, there's my own fault to this. For example yesterday when we were building the town square, the new guy asked us where are we, so I teleported him, but later on we started chatting with each other, nearly ignoring the guy, he didn't left accepted by the community and left. ( Just my opinion )
About the forums, we can still make it more popular there, there are plenty ways of doing that.
Outside Minecraft forums - Hundreds of possibilities again. Well most effective way would be casting Youtube series on the server. Or even simply making a trailer of it. Anything that would gain at least some popularity.