The interesting thing about whatsapp is that it works on feature phones too. It's not targeting smartphones vs. ipad users, but getting smartphones (all platforms), and features phones on a single messaging system for just $1 a year.

Sure I would kill for an ipad or a desktop version but does iMessage work when your friend uses Android or Windows? What about Grandpa using an old Nokia? How about those old friends who moved away to another country? Moving from the US to another country isn't very common, but it's fairly common for the rest of the world - born in Dubai, Schooled at Cairo, works in France is very likely.