David Gentzel is one of the top Facebook developers right now. He has launched a number of other applications that each have millions of users. His most recent creation, Happy Hour, has added close to 1 million users in under a week. If you want to figure out an effective way of developing viral applications, look no further than David’s applications. Happy Hour requires that you invite at least 5 people prior to entering the application. If there were any doubts about whether or not inviting users is a barrier to using an application, this should answer that question. Close to 1 million people (I’m sure it will be more than 1 million by the end of the day) have been willing to go through the invite process. The application is insanely simple as well. While I keep arguing that applications need to have robust features, David Gentzel keeps proving me wrong by releasing simple applications that rack up rapid numbers of users. All that you do in the application is send your friends drinks. You are given a daily budget and you can choose the type of drinks that you’d like to order. That’s all there is to it. If you want to buy your friends a drink, go grab the Happy Hour application.