http://phecoopwm6x7azx26ctuqcp6673bbqkrqfeoiz2wwk36sady5tqbdpqd.onion/posts/misc/constructing-a-hyperlink-to-share-content-onto-facebook.html
The various social networks often provide "share this" widgets that can be embedded into sites. 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).