Try to create engagement and loyalty through a drop campaign for your users. For instance, when a user signs up and does not complete the tasks he/she needs to complete before he/she can be fully reap the benefits of your service, use automated emails to get them to come back and complete the tasks.
1. User signs up but doesn't return. Remind them in 3 days or a week.
2. Ask them to upload their profile image if they haven't already.
3. Ask them to make the first interaction (like a message, upload an image, tag a picture, send another user a message, etc.)
4. Abandoned Cart - send an email to remind them to come back to make the purchase.
5. Just poke them - if they've been away for long enough, just send an email like "We missed you, come back and check it out!"
6. Feedback Email - They've done all the tasks required, now ask for their feedback!
There's a lot more, but in general, it's just to get your users to come back and perform the most important actions on your site/app.