But what can be done to have the users 'return' to the site/app (even every day if possible)?
Here are some examples of sites that have great retention: Facebook, LinkedIn, Twitter, Quora, GrowthHackers etc. Obviously, all the above web sites have awesome, fresh, (user generated) content - but there's something else common: 'The Hook'
With Hooks you'd basically try and get your users, over time, to get habitually participate and contribute. Most social media sites do this amazingly well, a few examples of such hooks -
LinkedIn's endorsements feature - these endorsements triggers create a retention loop wherein active users bring back inactive users
Quora would send a notifications every-time there's new content, a question, or an answer to a question, that's being followed, which brings inactive users back to the site
You'll need to look into your product/website to see the different possibilities of setting up a retention loop that will help drive users back to your website to contribute (& logoff) to return later as part of the loop. (round-and-round-we-go!)
Example: A blog space, could consider comments to drive the discussion. A gamified model could employ user badges or start ratings which can be used as triggers to bring users back to view any new activity involving it.
So, I'll leave you with links to some good reads on this subject (below).