Critical mass is always a huge concern for businesses that look to take advantage of the network effect.
My advice is to go where the traffic is, instead of trying to get the traffic to come to you. Can you enable your community or discussions on facebook? Are there existing websites that you could layer a community on top of?
That said, there are a few ways to avoid the "empty ballroom" syndrome:
Finally....build content and value for an audience in other ways than just the community/forum! Your product should be improved by the network effect, but not completely rely on it to succeed.
First you need a hot topic area that a lot of people are interesting in like... the in's and out of using Paypal.
Then you need some good Q&A forum software like.... StackExchange.
Then you kickstart it with some questions that you already know the answer and invite about 20 of your friends to participate early on so there is always some fresh material.