So the connection is spotty. There's a post right from WhatsApp directly saying they are having issues, so nothing to attribute to iOS/Apple at this point (at least not until they say everything is fine and iOS 7 or iOS devices in general still don't work with it after that).
There might be bugs in iOS 6 apps that determine connection from wifi to cellular that might be causing some connection issues. There is a change in the API noted in the release notes