I do think a lot more members fade away than stay. It just makes the forum seem active and friendly when they get hellos and welcomes. It's that whole first impression thing.
Hit gets it. A forum that doesn't invite new members is a forum with nothing but old members. Invite the newbs and educate them when at all possible.
I had heard of a Gracie Killer when I first joined but didn't know who Sakuraba was until my first 50 posts.