http://phecoopwm6x7azx26ctuqcp6673bbqkrqfeoiz2wwk36sady5tqbdpqd.onion/posts/misc/constructing-a-hyperlink-to-share-content-onto-facebook.html
However, these involve the user's browser requesting resources from the social network, and these requests are known to be data-mined in order to track users across the web and profile their browsing habits (to better sell ads). However, most social media networks do make it possible to create a share button which works via a simply hyperlink.