1. I'll concede the network effect, but if you are having trouble supporting - for example - 10,000 users with a .5% conversion rate, 100,000 with that same conversion rate isn't going to help you. You need to change the ratio, not just grow.

2. You'll need to find me some stats that back up the point that there is a relationship between the length of time a user is on a site and their commitment to purchase. If this is true, all websites should see increased per capita conversion rates over time. I don't think this is the case, but please prove me wrong.