In Windows 8, all you need to do is Share the content as a Share Source. This lets the user choose which app they want to Share your content with, as the Target. Maybe they don't want to use the default Twitter client, they'd rather use Tweetro. This mechanism lets them do that, while also keeping your app forwards-compatible with other apps and networks that haven't even been invented yet.
I wrote an article on how to do this here: