I agree with Mz that the most likely long term users aren't necessarily success stories. I think it's reasonable to expect that you're going to have significan

I agree with Mz that the most likely long term users aren't necessarily success stories. I think it's reasonable to expect that you're going to have significant churn. When someone stops using the site regularly after achieving their goal, though, they are still very valuable to you. Find a way to follow up with those folks each 6 or 12 months and you can hopefully gather data to prove that your model works. The people that have been the most successful may very well want to talk about it. Being able to say "5000 people have used Skinnyo to meet their weight goals and kept the weight off for 12 months" in big letters on the front page could help your conversion rate, and may even help current members stick with it.