WhatsApp was a game changer for people that had international contacts. Before WhatsApp, all the mobile networks (at least in the US) charged exorbitant amounts of money for international SMS and MMS.

Then, WhatsApp came and it was free and cross platform, compressed images and video to send them quick with decent quality, and free of spam. Absolutely perfect execution, in my opinion. If it was not owned by Facebook, I’d probably pay a considerable amount for it if it promised security and privacy.