I think you misunderstand viral. It may be virally shared, but it's not viral by its nature. A viral app is an app like skype or paypal, as they only get useful when the other person uses it. An app like facebook, you interact with the other person over the site, as the person who invites you is 'there'.

Your app is just a 'share this amusing link if you like' type site. It's not inherently viral app. Change it in a way that people want to invite people, and also to stay there. Then it will grow on its own.