On Net Neutrality: Are mobile operators paid to promote certain messaging apps?

In Trinidad, for example, on Digicel, one of the two largest mobile operators, using Whatsapp does not count toward your data usage. But any other messaging apps do. Observe the "extras", http://4g.digiceltt.com/en/pricing/4g-data-rates I wonder if this usage is being subsidized by Facebook. And if this practice occurs in other countries. Because in practice it is a blow to Net Neutrality.