Here's a great article, with examples from Foursquare, Instagram, Quora and Path, about how to hook your users into engaging with your app. In short, figure out which user actions drive engagement (e.g., for FriendFeed, it was "connect to 5 or more people"), and focus your design on encouraging this in new users.
The article also covers how this "design for engagement" needs to change depending on the kind of application you're building. For example, Instagram encourages you to follow more people so you see more cool photos in your stream, whereas Foursquare encourages you to follow people you know, because check-ins by strangers are boring.
