WhatsApp was an existential threat to Facebook in every country besides the US. They had the data to prove it, obtained from real users via FB's Onavo VPN service. Facebook Messenger 3.0 was to WhatsApp as Google+ was to Facebook (the product), and when the usage numbers weren't going the right way they gave up and bought them out before the gulf got even wider.
Textual messaging is a low-data-use (accessible to the cheapest phones with the smallest data packages) entrypoint to capture a person's social network so you can have other opportunities to capture them again and again with other services in the future. Facebook saw India as an especially huge burgeoning market at that time (hence Internet-dot-org / Free Basics), and afaik WhatsApp is ubiquitous there.
