And it did it all without spending a penny on advertising. Dropbox, the cloud-based file-syncing and file-sharing startup, is clearly one of this year's "it" girls among tech startups.

Dropbox has climbed from four million to 25 million users since January 2010. That's a 500% + growth rate in a little over a year. What startup wouldn't salivate over stats like that?

Dropbox CEO, Drew Houston, tells Fortune Magazine that 30% of his new customers are generated by their referral program. Users who refer a pal to signup get an extra 250 MB of storage for free.

Here's some more Dropbox stats to chew on:

- 200 million files a day are saved via Dropbox.

- Houston says 80% of Fortune 100 companies are now using Dropbox.

- Pull back the curtain at Dropbox and you'll find a staff of just 45 people. 

It will be interesting to see how this company navigates through its meteoric success.