When users share your content to a social network using the Open Graph protocol, e.g. Facebook, you can control the content that network will associate with the shared url.
One really nice attribute to control is og:image. Zesty will automatically detect the first entry in a page, pageset, or dataset with a field containing the word "image" and use it as the content value of og:image.
A common practice is to create a social_share_image field that will be used for social networks.
Facebook caches shared urls so if your adding this to pre-existing content you may need to use their Debugging Tool to update it.