You can include links to your social media accounts by going to the Settings page of your site. At the bottom there is a section titled "Social Media." If you select "Yes" for "Show Social Media Sites" social media icons linked to each site you specify will be shown on all pages of your site either in the header or in the navigation menu depending on your theme.
For each site you would like linked, enter only your username or whatever information comes after the site URL into each input box.
For example, for W&M's Facebook site you would enter: "williamandmary." The "http://www.facebook.com" (or whatever the social media site URL is) is automatically prepended to the value you enter, so there is no need to include that in the input field.