Pandora pays for mechanical rights, but it's a lower rate than Spotify pays, for instance.

Check out TuneCore's Music Industry Survival Guide. It's a useful resource.

http://www.tunecore.com/guides/sixrights

Edit: I'm wrong (and so is Tunecore). Parent is partially correct, Pandora does not pay for mechanical rights. They qualify as a PurePlay Webcaster. However, Interactive Streaming services, such as Spotify, do have to pay mechanicals in addition to recording and performance.