That's a somewhat different use-case though because the alternative (text-messages) always had some sort of price tag, and iMessage wasn't around when WhatsApp started.
WhatsApp also is free for a year, literally costs $1 per year after that, and is for phones, which generally have a fast payment process. The friction is a lot less!
There were free mobile clients for email, Skype, Google Talk, AIM, Yahoo, ICQ, etc.
True, but they're only extending an existing service you have to be a part of.