BigDoor's Keith Smith, Matt Shobe and Jeff Malek at the company's new offices in Seattle's South Lake Union neighborhood

BigDoor believes it has the keys to success to increase engagement on Web sites. And today the fast-growing Seattle startup — led by technology veterans Keith Smith and Jeff Malek — is taking a step to accelerate those efforts.

BigDoor has acquired OneTrueFan, a San Francisco-based startup which has developed a Web site widget that displays which readers are the most active at sharing content via social media channels such as Twitter and Facebook.

As a result of the deal, OneTrueFan’s five employees will join BigDoor, establishing a new beachhead for the company in San Francisco. That could be an important move since some of BigDoor’s biggest rivals — including BadgevilleBunchball and Gamify— are located nearby.

OneTrueFan is led by co-CEOs Todd Sampson — who previously co-founded MyBlogLog — and Eric Marcoullier — who previously co-founded online games site IGN.

In an interview, BigDoor CEO Smith described Sampson and Marcoullier as “great serial entrepreneurs” who will add a lot to the BigDoor team.

The OneTrueFan service encourages readers of blogs, gaming sites and other content-rich Web sites to compete to become the Web site’s biggest fan, thus driving deeper engagement.  There are thousands of publishers currently using the service, primarily small and medium-sized Web publishers who are looking at ways to showcase the biggest fans and drive deeper engagement.

With the acquisition, BigDoor now employs 31 people. The company — which raised $5 million from Foundry Group, Founder’s Co-op and others last summer — is already starting to push the limits on its new office space in Seattle’s South Lake Union neighborhood (located downstairs from TechStars and Founder’s Co-op).

What’s driving the growth? BigDoor is helping to lead the charge on a new Internet phenomenon known as gamification, basically the idea of adding game mechanics to Web sites. Essentially, that means that fans of sites are rewarded for their activities — perhaps retweeting a video or “liking” a story.

To date, Smith said that the BigDoor team has even been shocked with the results. For example, the company said that users who participate in the BigDoor loyalty programs are 300 percent more likely to return to the site than those who do not participate. Meanwhile, engagement levels for those who do participate increase by as much as 30 percent.

The acquisition of OneTrueFan marks another milestone. Smith says that it is the first acquisition in the emerging gamification industry.

John Cook is co-founder of GeekWire. Follow on Twitter: @geekwirenews and Facebook.

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