FanBridge has a fairly extensive platform that helps people like Lady Gaga manage communications with her fans via social media.

But for all of its technology, its most simple, an email called Social Digest, is growing the fastest. Today marks six months since the New York-based startup launched Social Digest.

The opt-in emails have six million subscribers, a number that’s gaining significant momentum. Of the six million, three million were added in the last month; that number is rumored to top 10 million by the end of the month.

“We’re tripling down bets on (this product) now,” co-founder Spencer Richardson told me.

Fans of artists, athletes, politicians, or celebrities can sign up to receive a regular email updating them with the most interesting recent social media activity from their favorite public figures. The emails are populated automatically according to which celebrities’ social media activity performed the best on their various social media outlets. They’re also populated dynamically in real time at the moment the email is open, meaning that — like magic! — an email sent yesterday, but opened today, might include Tweets and Facebook updates from the past 24 hours.

Social Digest emails have twice the average open rate and three times the average clickthrough rate of a regular email from an artist’s mailing list. (FanBridge manages influencer email lists that reach 150 million fans). The company also serves ads in its emails from brands; ads in Social Digest emails have a higher clickthrough rate as well.

Ninety percent of users are entertainers and public figures now, but brands are increasingly adopting Social Digest to push content from their social media accounts to fans, as well.

“We went after email because, quite frankly, it is the biggest headache for our clients and everyone knows that it’s a higher ROI channel. But its the biggest pain,” Richardson said.

The company raised $2 million in venture backing in 2010 from Founder Collective, First Round Capital, 500 Startups, Lowercase Capital, and SoftTech VC. That year it acquired DamntheRadio, a loyalty, rewards, and promotions company backed by i/o Ventures.

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