Probably the best blog post on determining your unit economics in a freemium business is by Andrew Chen here:
http://andrewchenblog.com/2009/01/19/how-to-create-a-profitable-freemium-startup-spreadsheet-model-included/
At YouSendIt we track (largely) the same key metrics and optimize the funnel to yield an increasingly profitable model.

I think even in a niche model you want your current users to share your content with like-minded friends and drive traffic to your site. From that point on it is the quality of the content that does most of the work but existing behavior (visiting, sharing, contributing) can be reinforced by loyalty programs. Some good data on this comes from airlines, credit card companies and grocery stores that see huge lift in revenue from achieving top of mind (reach for that Citi AAdvantage card instead of the other two you keep in your wallet).

And getting back to freemium / YouSendIt: these days getting a user to come back even one extra month means *millions* in revenue (we currently have 400,000 paid subscribers).

PunchTab takes these fundamentals and makes it easy to implement for the average site owner and large brand alike. We’re improving and optimizing every day and, like the businesses we serve, are focused on the full customer lifecycle. You can read blog.punchtab.com where we will have and will continue to share success stories.

If you have site with a niche topic and you produce quality content, then will using these type of tools really impact the volume of visitors? I’m curious if there’s evidence that this model is effective for all topics. And if so, is it sustainable — once the novelty is gone?