A new feature has come to Facebook’s largest cross-promotion bar, Applifier: retargeting, a tactic usually used by ad networks. With retargeting, developers will be able to send optimized messages to users who have encountered their game before.

Here’s an example use case. Applifier’s bar is essentially a line of ads, in which developers trade views on a one to one basis. With retargeting, you can make a user that has already clicked through your ad once see a different message — instead of “Try our game!”, it can be “Come back!” Regular players can see yet another message.

Retargeting won’t help developers get new users, but it will help them improve retention for existing players, according to Jussi Laakonen, the CEO of Applifier. Some users use the Applifier bar almost like a navigation toolbar, instead of returning to the front page to click through Facebook’s built-in bookmarks.

Applifier is working on more features beyond retargeting, Laakonen says; it’s just one of the first to launch. The company has good reason to try to differentiate itself. Since launching last May, the company has gained two strong competitors, in the form of Tapjoy’s AppStrip and Digital Chocolate’s VIP Games Network.

There are arguments for each network. One of Applifier’s has stayed the same since its launch: it’s independent, and untainted by big developers. AppStrip, as we pointed out early this month, has benefited from selling substantial traffic on its network to Zynga for CityVille, while Digital Chocolate gives a pure 1:1 exchange (Applifier takes 10 percent of traffic for its own revenue) but, presumably, uses the data from the network for internal purposes.

All the competitors will find fertile ground for expansion,  says Laakonen, but he’s sticking by his model. “As a publisher, the best company to work with is not just the one that has the largest reach, but one that has your best interests in mind,” he says. “We’re data neutral — if you’re a publisher on Applifier you don’t know where your traffic is coming from. We want to make sure people don’t get competitive info.”