That's "zero rating". Facebook specifically paid telecoms to offer that - it's just a B2B agreement with special network rule infra. A few countries banned it as anti-net-neutrality Zero rating still exists on airplane Wi-Fi when you can access Messenger without buying in-flight

Facebook’s plan to find its next billion users: convince them the internet and Facebook are the same

This is the story of Facebook’s rapidly unfolding plan to take over the world, or at least the world wide web. It’s a tale that’s been hiding in plain sight for years, and it begins with an explanatio...