Does your application have users? I hope so! Keep them around and active.
The best way to get users engaged, and keep them coming back is to use email marketing. You can create a sign-up workflow, move users to different workflows based on what they are doing in your app, and re-engage users that haven’t logged in for X months/weeks. Its very easy, so here is some code that connects to the MailChimp API and lets you add users to Lists.