As far as I know, Twitter and Facebook both read any URL submitted to them and reset the URL to their own preference. Even if a wp.me link is submitted, it would be changed, in my experience.
We’ve discussed adding the “via” text but have settled with not adding it by default. The question comes up: should that be “via @the_blog_owner”? Or “via @the_person_sharing”? The answer is different for different users/blogs.
Then you should make an update where we can customize it for ourselves. WE can determine exactly which one it is. We should be able to add hash tags and such if we want, too. That would be great if you added this.