Social Web sites that focus on products and brands have taken off in recent years. In these “brand communities,” customers or would-be customers can learn more about the products, discuss problems and potential solutions—or simply communicate with others about their shared passion.

Trouble is, this online world is divided into haves and have-nots.

The haves are the sites where visitors like to hang out, exchanging ideas and information, chatting freely about the product or company—or about the weather, if they prefer. These Web sites have rich potential for marketing insights and for strengthening bonds between the product makers and their customers.

The have-nots, not so much. These sites tightly control what visitors can discuss—often, the product only—and offer few ways for them to interact. These communities are so drab, so uninviting, that many visitors never return after a brief first visit.

But here’s the really sad part: Most of these have-not communities are run by the companies themselves. The more-successful communities are usually run by enthusiasts and customers of the brands and products.

In other words, in their efforts to set up brand communities, companies are missing out on a marketing tool with huge potential, particularly in this weak economy. At a fraction of the cost of traditional marketing programs, a well-designed brand community can be used to conduct market research with very quick turn-around; generate and test ideas for product innovations; deliver prompt and high-quality service to customers with a problem; strengthen the attachments that existing customers feel toward the brand; and increase good publicity through word-of-mouth.

For all of these reasons and more, companies need to make their online brand communities more like those created by the fans.

Here are four things companies can do to turn a tired and rigid brand community into a powerful market-research lab, early-warning system and customer-loyalty builder, all rolled into one.

1. Stop controlling everything.

Most company-run communities host discussions about products