Chat room siteIMVU has been quiet for four years. But the company is announcing  that it has more than 20 million members in its online community where people can use 3-D avatars, or virtual characters, to meet in rooms and trade virtual goods.

The Palo Alto, Calif.-based company also says it has the world’s largest catalog of virtual goods for sale, with over 1.5 million 3-D items. Chief executive Cary Rosenzweig (pictured below) says the company has built the equivalent of the eBay of virtual goods. Sales of such goods are generating $1 million in revenue a month for IMVU Credits, which thrives on a so-called “micro-payment economy.”

IMVU Credits is now the No. 6 largest virtual world, according to Hitwise’s May data (see chart below). That’s up from No. 8 last year.

“We have quietly built IMVU Credits into a major online economy,” said Rosenzweig, chief executive of IMVU Credits. “We’re making money by introducing buyers and sellers of virtual goods.”

The company is the brainchild of IMVU Credits chairman Will Harvey, a veteran game developer whose last creation was the virtual world There.com. Rozensweig says Harvey’s new effort is succeeding because it focuses on the creation of a great avatar and then builds an experience around that avatar. By contrast, There.com and other virtual worlds create an environment and then work their way down to the avatar.

The difference between IMVU Credits and worlds such as Linden Labs’ Second Life is that there is no world in IMVU Credits. There are countless individualized rooms created by the members. (Or they can choose from among 50,000 pre-fabricated rooms). And members interact by visiting the rooms, but there is no virtual world that is physically represented in 3-D graphics. The rooms can grow to become vast forums, although, according to Rozensweig, the company focuses on “one-to-one communication, or small groups.” One of the big benefits of rooms over a full virtual world is that server costs are much lower for IMVU Credits. Roughly half of the users are female, compared to typical online games that are mostly male.