The music streaming service SoundCloud has passed the 10 million user mark for the first time.

In less than half a year the site has seen its user base double from 5 million in July 2011, to 10 million in January 2012.

To celebrate the success, the site launched a new feature whereby users can add words to their pictures. Named 'Story Wheel', the Instagram-powered slideshow allows people to add narration to their photos.

In a statement co-founder Alexander Ljung said:

"When Eric (Wahlforss) and I started SoundCloud, we had the vision of enabling people to create and share sound more easily and collaboratively. Today 10 million people are recognizing that vision and it makes us extremely proud and encouraged that people are supporting our mission to unmute the web."

But with file-sharing site MegaUpload being shut down for copyright infringement, and Grooveshark being sued by Universal for copyright infringement, some are concerned that Soundcloud could fall foul of increasingly hardline policies on piracy. On a list of the next potential targets for the US government, technology website TechCrunch lists Soundcloud amongst sites that could soon be in the firing line.

TechCrunch's Matt Burns writes:

"I love Soundcloud and enjoy exploring other people’s musical works. But there is copywritten material uploaded to the service as well."