I try to figure out the same.
Maybe we have to add an unordered list with class “social” manually in header.php right before the closing </nav> tag.
<ul class="social">
<li class="twitter"><a target="_blank" href="https://twitter.com">Twitter</a></li>
<li class="facebook"><a target="_blank" href="https://facebook.com/">Facebook</a></li>
<li class="feed"><a target="_blank" href="https://example.com/feed/">Feed</a></li>
</ul>
Take a look at the html source of the page.
Maybe there is a better solution using a plugin, but this way the icons show up.
I can’t find any settings in the administration area for setting links to social media profiles.