For me it was the network--everyone in my phone contacts was already available to text. Skype hasn't transformed to the mobile world yet: I have to add people manually even now. If WhatsApp introduces video calling and release a serous desktop/web application I am sure it will rival all Skype-like applications.

However, all is not perfect--the voice calling sometimes work, and most often it doesn't. Purely on objective terms applications like Telegram work better for texting and groups, but the network effect will keep WhatsApp floating. Hope they will improve it soon.