Prior to March of 2012, many bands and musicians used third-party applications like BandPage, ReverbNation, and FanRx (formerly BandRx) as the landing page for their Facebook profiles. These apps enabled said bands to position their product (i.e. music) front and center on Facebook, which most bands now use as their Internet "headquarters."
Then in late-March, Facebook forced all of its Pages into the new Timeline format. As such, Facebook users cannot change their profile landing page. Instantly, BandPage et al were relegated to secondary page status. Not suprisingly, the change had a dramatic and unpleasant effect on BandPage and its peers, as traffic for the apps took a nosedive.
The affected companies have since struggled to adjust, providing their users with instructions for directing music fans to their respective apps. Because after all, it was still the only way to actually listen to songs on Facebook (one of the few things Myspace still had over Facebook).
But now, mercilessly, Facebook has pounded another nail into the proverbial coffin of apps for musicians by introducing the "Listen" button. This new button resides directly next to the Like button, and when clicked, music will start playing via Spotify, or whatever program is set up to play nicely with Facebook on your computer (examples also include Mog, Rdio, and Slacker Radio).
So now, with one click, you can check out a band's music on Facebook, the very thing apps like BandPage and ReverbNation were designed to do in the first place.
It's a smart move on Facebook's part, but it may be death knell for a handful of companies that happily rode the wave of Facebook's rapid growth.