I recently saw Bob Kraut present a paper on the mediating variables for Usenet participation. The study looked at how getting a reply affected the chance that a poster would return and post again.
For oldtimers who received no replies, 84% posted again. For oldtimers who did receive a reply, 86% posted again. For newcomers who received no replies, 16% posted again.
What's startling though is the effect getting a reply had on newcomers posting their first time. When looking only at newcomers, getting a reply increased their likelihood of posting again from 16% to 26%. That's a 62% increase!
So how do you really grow a userbase from the ground up? You make sure people have an audience right off the bat.