There are a few ways. The most effective ones include spending a budget.
1. You can find press that handles tech/app news and pay for an article. It will automatically be shared on their social profiles and you will get exposure.
2. You can do the same with influential bloggers in the same area - tech, apps, etc. No idea what your app is about, you need to research the blogosphere to find matching people.
3. Gain followers yourself and at some point pitch your app with an explanation of what it is.
4. You can create games, promotions and other goodies and share them with an audience. If you have created a good campaign, the users will do the rest.