Where I live (Austria), Facebook messenger does not have the same reach as Whatsapp.
Pretty much everyone with a Smartphone has Whatsapp, but there are lots of people without a Facebook Account. These are mostly older people. There are also people (like me, but I know several others) who do have a Facebook account, but don't use it on their phone.
The really nice thing about Whatsapp is the installation experience. Just download the App from Google Play/Appstore. Start it. Confirm your phone number. And you can communicate with all your friends. Facebook is much harder in this regards. Signal is similar, "but network effects".
Pretty much everyone with a Smartphone has Whatsapp, but there are lots of people without a Facebook Account. These are mostly older people. There are also people (like me, but I know several others) who do have a Facebook account, but don't use it on their phone.
The really nice thing about Whatsapp is the installation experience. Just download the App from Google Play/Appstore. Start it. Confirm your phone number. And you can communicate with all your friends. Facebook is much harder in this regards. Signal is similar, "but network effects".