Online petition site SumOfUs rang in the new year by celebrating a fresh milestone in the life of the two-year-old site: It passed 2.5 million members, doubling its membership total in just 15 weeks.

High-profile campaigns targeted at companies such as Google and Apple helped to raise the site’s profile, said SumOfUs director Taren Stinebrickner-Kaufmann, who also pointed to major victories such as a push to get Trader Joe’s to sign a fair labor agreement for workers in its supply chain.

The petition site behaves as much as it can like a tech start-up. Stinebrickner-Kaufmann said that SumOfUs uses the “minimum viable product” model. The strategy, originally created for Web applications, tests the waters with a small, focused product and devotes resources only to the parts of the product empirically proven to work.

Marrying that tech methodology with its progressive organizing, SumOfUs thoroughly tests users’ reaction to campaigns to find what resonates most with members before it throws the full weight of the organization behind a petition. Stinebrickner-Kaufmann said that the site always gets feedback from a test group of 10,000 members before deciding to go through with a complete campaign.

Though, sometimes, SumOfUs users actually move so fast that simply sending alert e-mails to that test group can get the desired result. When the site recently decided to ask retailers TopShop and Asos to make sure that they responsibly source the angora fur in their products, SumOfUs users rallied enough support to get the companies to change their practices before the site itself could fully ramp up its efforts.

Sites such as SumOfUs are able to be so effective, Stinebrickner-Kaufmann said, because they can provoke rapid response on a global scale, even for the largest companies in the world. She said that the most successful campaigns are targeted at private-sector companies that react to customer outcry.

“In a networked world, a global economy, every person in the world is a potential Google user, every person drinks Coca-Cola or Pepsi -- that’s the thing that, from the technology perspective, that’s the really revolutionary aspect,” she said. “Giving people the opportunity to do that across borders was not possible before the Internet.”

And while SumOfUs is excited about the rapid growth it’s seen in the past two years, the organization is also very careful to make sure that it remains a platform for petitions that support its roots in progressive ideology. Stinebrickner-Kaufmann said that SumOfUs is very targeted about how it grows, comparing itself to start-ups like Pinterest that have focused heavily on protecting its core brand even while dealing with rapid growth.

That’s meant being careful about the petitions it promotes, and that in turn, Stinebrickner-Kaufmann said, tends to attract exactly the kind of people organizers want for the site.

“The nice thing about it is that it’s sort of chicken-and-egg -- you build the membership that responds to what you do,” she said.

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