In my experience, it's less than 0.5% on average, and in some cases, far less. However, I've found you can get better than average share rates by bluntly asking users to do so - with a lightbox after they've scrolled to the bottom of the page (indicating they've actually read the entire article), for instance. You can do the same if a user watches a video to the end, etc. You're not allowed to directly incentivize Facebook sharing, however you're free to strongly encourage it.
As with many things in life, it turns out that you won't get what you want unless you ask for it.