I'm afraid we haven't implemented any kind of count that would allow you to find how many people have shared a post by clicking on the Share button on your site. That's something we've added to WordPress.com, but we haven't ported it to Jetpack just yet. Keep an eye on our changelog, we'll mention it there as soon as we do!
You won't be able to find out who shared your post, though; Facebook keeps such information private.