<ul class="external-nav">
  {% for socialButton in site.data.social_buttons %}
  <li><a class="fa {{socialButton.name | downcase}}" href="{{socialButton.url}}"><span class="screen-reader">{{socialButton.name}}</span></a></li>
  {% endfor %}
</ul>

<ul class="external-nav">
  {% for socialButton in site.data.social_buttons %}
  <li><a class="fa {{socialButton.name | downcase}}" href="{{socialButton.url}}" rel="me"><span class="screen-reader">{{socialButton.name}}</span></a></li>
  {% endfor %}
</ul>

Add rel="me" to social buttons
Add rel="me" to social buttons