We’ve been following apps developer NoiseToys for a while now: it’s the company behind the Hitmaker music discovery app. Now it has a new thing: an iPhone app called Playground – Rock Your World. It’s the latest in a steady flow of apps promising social music. “Listen to music on radio stations DJ’d by your friends,” explains the App Store listing. “Create your own radio station and play your favorite tunes. Like or dislike songs as you hear them.” Oh, and: “Connect with Spotify and choose any of 15 million songs!” We’re assuming this means Playground is the latest app to use Spotify’s mobile API, following recent releases SpotON Radio and Soundrop.fm. For now the app appears to be US-only.