Just a few months after the mobile Q&A company Opinionaided rebranded itself as Thumb, it’s already seeing significant engagement gains — so much so that its users are twice as addicted as Pinterest’s.

Thumb is on such a tear that it has managed to tempt one of its own investors away from the cushy life of venture capital. Hadley Harris (below), former head of business at Vlingo (which was recently acquired by Nuance) and a founding partner in NYC’s ENIAC Ventures (the initial seed investor in Thumb), is now joining the company in the newly created role of chief business development officer.

“I think [Thumb has] the opportunity to build something special here … it was a unique opportunity to join,” Harris told VentureBeat in an interview last week.

Harris told me that Thumb’s users are now spending an average of 3 hours and 50 minutes on its mobile app every month, up from three and a half hours back in December. That’s engagement second only to Facebook. Pinterest, the much-hyped online image-sharing site, meanwhile only sees around 97.8 minutes of engagement per month, according to figures from ComScore.

The Thumb app allows you to ask questions and get instant responses from the company’s highly engaged user base. You can also give your opinion on questions from others. Back in December, users were seeing around 70 responses on average to their questions, a number that’s likely higher now.

It’s simple, but addictive, and that seems to be the key to Thumb’s success.

It was Thumb’s crazy amount of user interaction that sparked Harris’s interest. After he led Vlingo to 10 million users and a “huge exit,” he stepped down from the company and was planning to settle down into the life of a VC, focusing on mobile apps and services. But after serving as an adviser to Thumb (back when it was known as Opinionaided) for years and seeing their massive engagement numbers, he couldn’t help but climb aboard.

“This can scale and become a full-blown social network,” he told VentureBeat, echoing what we heard from CEO Dan Kurani in December. “Opinions are more valuable than interest,” Kurani said at the time, referring to Facebook’s emphasis on user interest.

As head of biz dev, Harris will focus on growing Thumb’s user base. He’ll also be working on strategic partnerships. Brands, for example, could be interested in getting a rush of instant opinions from consumers.

Thumb is based in New York City and has raised around $5.5 million across two rounds of funding.

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