Certainly four key ongoing activities are:
- Push out to new users
- Engage with your base (those who try, those who dip in, those who become regular)
- Measure everything you can
- Split test different ways of working
One observation I'd make from your website is that you're making people provide email addresses before you show value. That may deter some people who may like your service from getting started; but it also means that you're getting email addresses from people who have no idea whether what you offer will suit them.
So far, you have 25% of the returners you wanted. Well, that's a start. And you can work with them. Maybe it'll turn out that you need 600 sign-ups to get 25 regulars.
Now go to work with the non-returners. Take a third of them and offer them some help. Take another third and just send them a reminder you exist. Take another third and give them a questionnaire.
And that way, you'll start to find what users are getting out of your service, what they're missing, and how you can improve their experience.
I think you're trying to solve a really interesting problem. I really want news that finds me! Best of luck.