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Dropbox_logoIt has been a good few months for Dropbox, a startup that backs up and synchronizes your files — after crossing the 1 million user mark seven months ago, its audience has now tripled to 3 million. While publicizing those growth numbers, the San Francisco company also revealed that it has raised $6 million in a first round of institutional funding.

GigaOm first reported the funding based on a regulatory filing, but much of the information in that initial report was wrong, Dropbox said. For one thing, the company actually raised the money more than a year ago, in October 2008, but didn’t share the news at the time. Dropbox said it stayed quiet because it was negotiating for the URL and didn’t want to broadcast how much money it had, plus the company’s executives were more interested in building the product rather than drumming up publicity.

The round was led by Sequoia Capital, with participation from Accel Partners.

As for that growth, chief executive Drew Houston said Dropbox’s success has been driven to a large extent by its simplicity — you don’t have to learn a new interface to use the service, you just move files into Dropbox folders and they’re automatically backed up and synchronized across devices.

“Making sure to nail the experience for mainstream users is really our focus,” he said.

Over the next few months, the company will be rolling out more features for sharing and collaboration, he added. Dropbox also released an iPhone application in September that has been downloaded more than 100,000 times. The service is available in free and pay versions, and although Houston declined to offer specifics on the paid side, he said it’s “ramping up along with everything else.”

Prior to last year’s round, Dropbox raised a seed round of $1.2 million in convertible debt from Sequoia and Amidzad Partners. (Amidzad has also invested in VentureBeat.)

Competitors include Syncplicity and SugarSync.

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