The last.fm blog:
On February 15, the radio service built into Last.fm mobile apps and on home entertainment devices will become an ad-free, subscriber-only feature.
Last.fm Radio will remain free on the Last.fm website in the US, UK and Germany and for the US and UK users of Xbox Live and Windows Mobile 7 phones. We’ll also continue to offer radio for free via the Last.fm desktop app.
That sound you heard was the remaining last.fm users deleting the app from their phones and installing Pandora instead.