Not all sites come with Font Awesome installed. If your site started with a dataset called "Awesome Social Links" then your site came installed with Font Awesome V3. So that uses the older syntax of class="icon-flag" instead of the newer class="fa fa-flag." If your site came with that set, there should be snippet HTML file and LESS file that come with it. You can update the markup and styling in those files.
If you want to add Font Awesome to your site, their web site can help you get started http://fontawesome.io/icons/.
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