It's important to stress that Podbabble doesn't require accounts to view discussions - in fact, you can even comment anonymously without creating an account. If you click on any of the comments on the homepage or choose a podcast via search you'll immediately taken to its discussion page without any barriers.

Create content yourselves to fill the void until it grows.

When you say "it's a chicken and an egg problem" you're spot on. That's the problem for every new social network or user based platform trying to launch. Until you have a ton of users and content I would argue it's the only problem worth worrying about.