Facebook has deprecated access to people's “usernames” via API
developers.facebook.com
developers.facebook.com
"You must not incentivize people to use social plugins or to like a Page. This includes offering rewards, or gating apps or app content based on whether or not a person has liked a Page. It remains acceptable to incentivize people to login to your app, checkin at a place or enter a promotion on your app's Page. To ensure quality connections and help businesses reach the people who matter to them, we want people to like Pages because they want to connect and hear from the business, not because of artificial incentives. We believe this update will benefit people and advertisers alike."
Together with the recent Page changes that require paid promotion and extensive engagement just to reach fans with simple posts, I won't be surprised to see artists seek out other opportunities for promotion.
I already see young people (college and below) abandoning Facebook in droves, except to crosspost a selection of their instagrams. Sure, there is still a huge market of existing music fans, but with few new adopters and artists Facebook will soon lose out on what used to be a main channel for music industry advertisement.