File-sharing service recently shared some impressive numbers with us: The Campbell, California-based company surpassed 30 million unique users in the past 12 months. The news prompted its PR team to send out a self-congratulatory note to journalists featuring the provocative subject line, “Dropbox who?”

The goosed growth comes on the heels of YouSendIt’s partnership with Yahoo Mail, which helped add a million users in roughly two months. But the uptick also comes at a difficult time for file-sharing services, which have been joined by more and more competitors eager to enter the lucrative cloud space.

From hot startups like and Dropbox to industry giants like Amazon and IBM, the cloud is increasingly becoming a large part of the lives of consumers and integral to enterprise clients.

With 60% revenue growth in 2010, however, it appears YouSendIt is defending its market position well in the wild, wild west of the cloud industry.

Still, while YouSendIt has chosen to highlight total unique users, the real proof of a company’s success is its registered and premium usership. Currently, YouSendIt has 400,000 paid subscribers and 18 million registered users–by comparison, Dropbox boasted of reaching 25 million registered users in April.

In other words, it’s cloudy in Silicon Valley.