yeah, but whatsapp was the first to become really popular. When facebook bought it, it was already the most used app, at least here in italy (was like the most expensive acquistition ever made at the time). After the acquisition, the original founder of whatsapp joined with 50 million $ to the development of a little niche app noone uses called "chatsecure" by the cryptographer Moxie Marlinspike, then they renamed it to signal and started "recreating whatsapp but open source". Signal started late and always been many steps behind whatsapp in every features (except implementing signal protocol for cryptography).