You can see this live in the promotions bar on the Street demo.
Here’s an example of a footer with social links enabled:
You can see this feature live in the footer of the Ecstatic demo.
To set social links up, put your social media page’s URL into the matching field of your social media settings.
For example, a YouTube channel URL would look something like https://www.youtube.com/user/shopify, where shopify would be your own username.