Find people on Twitter who are experiencing a problem that your app can solve. For example, if your app brightens photos, search on twitter for ["too dark" camera] (without the brackets). You'll quickly find people who could use your app. Reach out to them by replying, making sure to express your sympathy, and offering them your solution. With a little work, you can quickly spot "influential" Twitter users, and focus on them, making sure to maintain your relationship with them and sharing their success on your site or through your Twitter accounts. You can use Twitter to convert frustrated people into happy customers, and then share their story with your followers.