Yes, everyone I know outside the US uses it. If I understand correctly years ago in a lot of countries SMS was either very costly or unavailable, while WhatsApp was free.
Also, WhatsApp allowed people to send pictures and stuff. Effectively, it has replaced SMS, MMS, and MSN Messenger. There was a brief period where people would still communicate via Facebook chat on PC, but once smarphones became prevalent WhatsApp won the battle (except in the US, which is something I've never been able to understand)