These guys are pretty impressive, for a company no one really knows about or hears about.
Serious question: who uses it and why? :)
Someone on HN mentioned before that some of the folks behind WA were ex-Yahoo! coders. So I suspect scaling is nothing new to them.
Everybody who I know (including myself) uses WhatsApp to text people who are travelling, studying abroad, or otherwise living overseas but don't want to pay an arm and leg for international SMS.
It's just a text message replacement. It's essentially iMessage but cross-platform.