Re-targeting was an aspect of user acquisition that was cited by at least one industry insider as being a key trend for 2013.

But clearly that's not happened as at the App Promotion Summit in Berlin, Trademob's head of sales Florian Lutz pointed to re-targeting as being a key trend for 2014.

"A lot of money is being spent in mobile advertising, and it's only going to grow, but most of these users never come back to your app," Lutz said.

"Users bring you money, not downloads."

Re-opening doors

His argument is that inactive users are the industry's biggest problem.

"25 percent of users open an app only once, while 69 percent of users open an app less than 10 times," Lutz explained.

His solution is that developers need to think much more about the lifecycle of their product, not just about the launch period, which is where the majority of marketing is currently spent.

"CPI is so 2013," Lutz stated.

Putting figures on this, he said that a recent Trademob campaign demonstrated that it was 62 percent cheaper to re-engage an old user compared to buy a new user, although this was for an e-commerce app rather than a game.

Another example was getting a 150 percent better conversion rate with re-targeted users.

"We also need to work on new metrics," Lutz said, suggesting tracking the cost of a user re-opening an app would be a good place to start.