When you hire Spark PR, Appency or Appular your are not buying followers, but real connections and expertise. I haven't used any of these services but at least from calls and proposals they made it is much more about experience from 100+ launches and personal connections. Rember you will need at least $10 000 which is for indie dev really a lot of money. But PR is not about quantity, it is about true relations and in our "App Store world" about personal effort and engagement. And good product
So if you have something truly great and no cash, you can still make a successful launch!