Web: Sharing a song should be simple. If you are listening to a track on Spotify, you should be able to share it with a friend easily, without them needing to be a Spotify user. Bop.fm makes this possible by cross-linking songs between different services, so that if you share a Spotify link with someone who isn't a user, they'll be able to hear the same song on other services such as Deezer, Rdio, SoundCloud or YouTube.
You know what the best part about Bop.fm is? It just works. Using my Spotify account in the service, I searched for a song and played it perfectly. Then I copied the URL and shared the link with a friend who isn't a Spotify user. Bop.fm immediately recognised that and found the song on YouTube, playing that instead. Perfect.
You know what the worst part about Bop.fm is? It's completely reliant on you hooking up your Facebook account everywhere. So Bop.fm logs into your Facebook, which needs to be connected to your Spotify, Rdio and other services. This reliance, instead of letting users sign into all those services individually with a new Bop.fm account, is bound to be a deal-breaker for many users, and understandably so.
There's also a quality control issue at times. For example, searching for Queen's "Under Pressure" on Spotify gives you a perfect quality studio version. Share that link with someone who doesn't use Spotify and you get a low-quality version on YouTube or a live recording on SoundCloud. Either way, while it's the same song, it's not the exact same track, which audiophiles might find disappointing. But for casual listeners, this isn't such an issue.
If you have a Facebook account that you use with many services, then Bop.fm is one of the best ways to share music with people. It will always give you a way to listen to the song on different services. But you do need that Facebook account, which is just so sad.