Vendors can add their social media accounts to their mini store pages, if the marketplace admin enables the social media feature for vendors.

To enable the social media option for vendors you will need to activate the vendor privilege "Edit custom vendor settings" in vendor profiles for existing vendors
/admin.php?p=vendor&mode=update&aid=23

For new vendor registrations you must enable it in the "Vendor Default Privileges" in
/admin.php?p=admin_vendor_defaults

After the privilege is activated vendors can add their social media accounts through their panel under the "Settings" drop down menu.

The social media option is only available through the vendor's backend dashboard