We see tons of app updates come through the App Store every single day, but it's very rare that one of those updates removes functionality rather than adds it. The latest update for Bump for iPhone does exactly that, unfortunately. Bump is an app that came out very early in the App Store's life, and its core feature was that you could "bump" with another iPhone user to share contacts. Over the years, Bump has added more and more sharing features, to the point where it can now share files, work with Dropbox and even connect up with a Mac directly.
Last year, the company added the ability to "share" music from phone to phone, basically by sharing the song's information, and then by looking that song up on the target phone across YouTube and iTunes. But that feature no longer works, as per the app's latest update. It could be that Apple didn't like music being "shared" across phones without their standard cut being paid, or it could be that Bump just didn't think the code was working as well as it should. At any rate, you can't share music in Bump any longer.
The good news is that if you have a version of Bump that will share music, you can simply keep that version by not upgrading to the new one, and you can also keep the functionality. You can even drag the app out of your iTunes library (if you've installed it from an iTunes client), and save it elsewhere, in case you ever accidentally upgrade. True, this one feature may not justify having to remember not to upgrade the app every time (and you'll miss out on any future Bump features), but if it's something you depend on, there are ways to keep it active.