At least here in the UK SMS' are usually free (or very cheap) but whatsapp is popular since it's very easy to use, allows group chats/voice messages and works w

At least here in the UK SMS' are usually free (or very cheap) but whatsapp is popular since it's very easy to use, allows group chats/voice messages and works well when the connection isn't great. But mostly because it works on virtually every platform (even some feature phones), so you can assume that basically everyone has it.