FORTUNE — On Sephora’s Beauty Talk community, an online forum for fans of the company’s makeup and skincare products, katie1724 is a “Hall of Famer.” With more than 4,400 hearts (the community’s equivalent of the Facebook ‘like’), she posts and comments five to 10 times per day. It’s routine for katie1724 to offer insight on everything from shades of blush to the benefits of seasonal face creams.
Outside of Beauty Talk, katie1724 is Katie Majersky, a 32-year-old legal assistant from Pittsburgh who adores all-things Sephora. Since adopting the brand while living abroad in Paris (Sephora originally started in France in 1970), Majersky uses the Sephora community as both a creative and social outlet. She spends about $100 per month on the company’s products, but that’s not why “superfans” like Majersky are finding themselves on the radar of their favorite brands.
Companies are capitalizing on their most engaged fans, and many online communities receive their credibility and utility from the company’s most active consumers — not company employees. These superfans answer questions about products, post insightful blogs, and even provide companies with valuable marketing and product feedback — all without collecting a fee. “We used to have a board called ‘unanswered questions,’” says Bridget Dolan, VP of interactive media at Sephora. “We had to switch that because our superfans just took it as a personal challenge to always answer the questions so quickly.”
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At LEGO, executives are using their online communities to generate new ideas for LEGO sets. (The company is currently producing a LEGO version of the DeLorean from Back to the Future, an idea generated through its Cuusoo online community.) On a separate LEGO community geared toward younger fans, JayZX535 averages more than 3.5 hours on the site every day, and read more than 68,000 posts between November and April. More importantly, JayZX535 posted more than 1,200 times, providing the community with a steady stream of fresh content at all hours of the day. Of the site’s 10,000-12,000 active monthly members, roughly 1% of them fall under this “superfan” definition, says Mark Fothergill, head of LEGO’s childrens’ community.
Sony (SNE) began its online fan community in November but spent years prior connecting with fans through Facebook (FB). In 2009, a group of unhappy customers started a Facebook page titled, “I Have a Defective Sony TV,” and execs from the company acted quickly. “I thought the first thing I would get from them would be a letter from their attorney,” says Bill Giedraitis, 53, who started the page when he had issues with his television. “Instead I got a phone call saying, ‘The new president of Sony would like to talk to you.’ That was kind of a shock.” The conversation with Sony’s execs turned Giedraitis from an angry customer into a superfan. He patrolled the Facebook page multiple times per day, answering questions and dousing complaints as opposed to fueling them. In its heyday, more than 10,000 members used the page to express frustration toward Sony, essentially giving Giedraitis a second full-time job. The difference? “I’ve never taken a dime from Sony for doing this,” he says.
Over time, the page has declined in activity, and brands like Sony are moving away from Facebook for fan engagement, where questions quickly disappear down the timeline and the possibility for deep conversation is stifled, says Phil Petescia, vice president of customer relationship management at Sony. Many of the new forums are powered by Lithium Technologies, a San Francisco-based company that helps brands build online communities directly into their home pages. The process allows users to engage directly with other die-hard fans and gives brands a database of questions and resources that they own and operate.
Superfans like Majersky and Giedraitis don’t just provide a loyal service to their favorite brands, they also bring a level of credibility unmatched by brand employees, says Lithium CEO Rob Tarkoff. “An old-school CEO or manager would say, ‘Well why don’t we just hire those people,’” he says. “Well once you hire them you change their relationship to everybody else in the community. Now they’re just a marketer.” A 2012 Forrester study found that 46% of people trust consumer-written reviews, while only 15% trust social media posts directly from brands. And while this may not constitute as a superpower, in the world of business, it provides superfans with a pretty big stick. The value of superfans, says Petescia: “It’s incalculable.”