Yes, truly WhatsApp was the first of its kind. It's all the communication of sending a letter through the mail, except delivered electronically - one wonders why they didn't call it "electronic mail", or perhaps "e-mail" for short.
The group chats it offers are another huge innovation - for the first time, people were able to chat with each other by relaying their messages across the internet. Truly a marvel.
Personally, I divide the internet into two eras - "before Whatsapp", when there was simply no primary convenience of any sort to be found upon the internet and all users were deeply encumbered by bounds; and "after Whatsapp", when I and others can communicate, conveniently, via the internet, because of WhatsApp, boundlessly.