Good post - an additional feature you could consider incorporating within your application is, adding something to your application which helps to 'spread the word' - such as (opted in) automated tweets (see tinychat - http://search.twitter.com/search?q=tinychat) or sharing widget (such as plaxo - http://www.plaxo.com/api/widget) where you can share with friends email addresses. You could also add the ability to share the link or your application/product through Digg, Facebook, Reddit, Delicious, Stumbleupon (which you now see on most blog posts).