You answered your own question: *In the US*. WhatsApp is huge outside of the US....

tjbiddle on May 12, 2015 | parent | context | favorite | on: Verizon to Buy AOL for $4.4B

You answered your own question: In the US.

WhatsApp is huge outside of the US. If memory serves correctly, SMS has always been expensive and MMS never really took off - once the data messaging platform came to be WhatsApp really served that market and has done well because of it.