Habbo has built one of the most successful virtual worlds with its simple, two-dimensional animated avatars and hotel rooms that users can decorate. Now it’s tweaking its monetization strategy by adding a new kind of currency for its 10 million unique monthly visitors.

Sulake, the Finnish company that operates the Habbo world for teens, said it will have two currencies. Members can still pay real money for “credits” to buy items such as virtual furniture, clothes and accessories. The new currency, “pixels,” can be used to buy virtual goods at a discount, rent items, or use a range of special effects.

This shows that the virtual goods market is hot in the game industry and that companies can use virtual currency in a variety of ways to influence user behavior.

The company will award pixels to those who log in often and otherwise interact frequently, showing that it’s willing to reward users for spending time on the site — since that translates into added revenue for the company.

Users can also earn pixels by completing achievements inside the world and by joining a subscription-based Haboo Club, or members club. Pixels are meant for different kinds of purchases, particularly temporary ones. Pixels can be used to rent hover boards, on which users can fly around the virtual world.

Habbo is available in 32 countries, and users have created more than 117 million Habbo characters. The service competes with a variety of virtual world and virtual space companies, such as Gaia Online, IMVU, Teen Second Life, and WeeWorld.

[update] Teemu Huuhtanen, president of Sulake North America, said that the company started researching the issue in 2007. Users wanted to be rewarded for loyalty and time spent on the site. The company started testing the pixels in the summer of this year, first in the United Kingdom. It worked out so well that the company expanded it worldwide.

Huuhtanen believes that revenues and transactions overall will be up as a result of the introduction of pixels. At the moment you can’t convert between pixels and credits.