Say No to Any Ulterior Motives: Many Facebook apps are shady, and aren't what they seem to be. These apps are trying to get users to take an action other than using the application. They could be trying to get people to click on an ad or to buy something. You won't see long term results going this route. When you build your app, focus on making something that people want to use. Ensure that your app doesn't deviate from what it's created for, so that it doesn't look shady or try to the get the users to do something else that they don't want. By providing real value, you can attract a large audience as they see that your app is honest. Your first app should not be action oriented; save that for later.
The more unique your app plus the more value it is perceived to be, the greater chance someone will send it to their network for all to have a look. You never want to do anything in your app that will prevent it from being shared with friends. However, don't take this too far and force your users to share your app just to use it, this will certainly backfire. It is really easy to make something of such value that they will want to do it all on their own. It always goes back to the same thing about offering something worthwhile that fills a need with people.
Time the Release: It's a good idea to learn the usage patterns of the networks where you plan to release your app before the launch date. So if you plan to start with your Facebook friends, learn what their habits are. Asking your friends to use and add your app while their friends are online raises the chances of your app catching their attention. Quickly breaking into social circles is the key to the success of your app.
So this is all really very basic, but there is not a lot to it - but the rules of engagement do matter a whole lot.