Below an article in your mailing, you will see the social media buttons and the option to Share. So, if the recipient of your mailing clicks on the Facebook icon for example, the article will be shared through the Facebook account of the recipient. Note that it will not be shared on your organization's Facebook page.
The description that you entered in the social media pop-up is included in the update that your contact shares, so be careful with what you write in this description.