ANSPs pay for the service, how much is negotiable. FAA have a "letter of intent" with Aireon, annual fee is in the region of $100 million.
Along with ANSPs the bulk of Aireon's customers will be airlines and airports. (There are many more of them than ANSPs).
Airlines in particular use position data to monitor routes and continuously improve operational efficiency. In the future global flight tracking may also be mandated by regulators. Airlines currently source position data from aggregators like FR24 and Flightaware which blend ADS-B sources with real-time ANSP data, or from service providers using ACARS (ADS-C) feeds.
As ve3id mentioned, Flightaware is an Aireon partner so they will have first mover advantage of this new service, with Qatar Airways set to become their launch customer.
By the way Aireon is majority owned by NAV CANADA.