I would not typically call it 'the power' of social media because this has always been how Taylor Swift communicates with everyone. It's simply the most immediate way to respond to something or have your thoughts reach people widely.
As far as a "PR stunt", it's certainly not the kind of PR that Apple would have wanted for several reasons. But I feel even the suggestion that it might have been a PR stunt shows a lack of knowledge about the subject matter--it's a very real issue for the music business and thank goodness there are people like Taylor Swift willing to discuss it and inspire change. Songwriters and artists are affected by Apple giving away their music for free. We are professionals and should be compensated for our work, period. Songwriters and artists have not received adequate compensation for their work for a long time across multiple platforms--everything from having their music played without compensation in stores and restaurants to downloads, something which existed long before Napster started offering people's music for no compensation years ago when the internet caused a profound change in the music business. And as people began to pay for streaming, we were told that money was being held in escrow till the laws were settled. The laws hadn't caught up to what was happening online. And what has happened to all that money from those first early years ? It didn't reach the songwriters and artists and never will.
So for Taylor Swift to come out and complain that it was unfair and would penalize musicians, writers and artists in different ways, was the good will of an artist who knows she has the power to affect. And she did it in such a gracious way, praising Apple for so much at the same time. Many artists and writers had complained as she said, but it takes someone like one of the biggest stars in the business to shame one of the biggest, richest companies into fairness. And she welcomes that role and uses it well. Those of us who work in the music industry and are affected by downloads with no compensation are grateful she's willing to speak out and help make for change.