This is neat, but it seems to violate Facebook & Twitter policies around content sharing.
For Facebook, "Ensure that all content in the user message parameter is entered by the user. Don’t pre-fill. This includes posts, messages, comments, and captions.". In the example, you suggest pre-filling content. You also append a URL at the end of the post.
Same for twitter... "If your application allows users to send Tweets or other content to Twitter, show the user exactly what will be published." and "Your Service should not use a shortened URL to mask the true destination"