Just implemented sharing via SMS, Mail Twitter and Facebook with the from the WP8 SDK provided ShareStatusTask:
Whatsapp does not seem to be supported here - do you guys know a trick to get around this?
Technically no, not yet, but something may come in the future using Uri Schemes.
For now, it seems that you can only run the WhatsApp app by doing this:
await Windows.System.Launcher.LaunchUriAsync(new Uri("whatsapp:"));
but on iPhone, you can use this form of Uri
whatsapp://send?text=Hello%2C%20World!
so I expect the same functionality to appear in the near future in WhatsApp for Windows Phone. There's no reason why they wouldn't implement it.
In fact, maybe they already have, but I haven't found it publicly documented. See on Nokia Developer website for other Uri Schemes
My understanding is that you can only make your App available as a means of photo sharing. (See here: http://msdn.microsoft.com/en-us/library/windowsphone/develop/ff967563%28v=vs.105%29.aspx)
There is no way to share to a particular third party App, unless it has "Extended the share picker" as seen at the link above.