You cannot mix a HTML element in Magento's XML. The XML parser for the current link class takes a string.
If you look in the class Magento\Framework\View\Element\Html\Link\Current
/**
* Render block HTML
*
* @return string
*/
protected function _toHtml()
{
if (false != $this->getTemplate()) {
return parent::_toHtml();
}
$highlight = '';
if ($this->getIsHighlighted()) {
$highlight = ' current';
}
if ($this->isCurrent()) {
$html = '<li class="nav item current">';
$html .= '<strong>'
. $this->escapeHtml((string)new \Magento\Framework\Phrase($this->getLabel()))
. '</strong>';
$html .= '</li>';
} else {
$html = '<li class="nav item' . $highlight . '"><a href="' . $this->escapeHtml($this->getHref()) . '"';
$html .= $this->getTitle()
? ' title="' . $this->escapeHtml((string)new \Magento\Framework\Phrase($this->getTitle())) . '"'
: '';
$html .= $this->getAttributesHtml() . '>';
if ($this->getIsHighlighted()) {
$html .= '<strong>';
}
$html .= $this->escapeHtml((string)new \Magento\Framework\Phrase($this->getLabel()));
if ($this->getIsHighlighted()) {
$html .= '</strong>';
}
$html .= '</a></li>';
}
return $html;
}
You will see $this->escapeHtml((string)new \Magento\Framework\Phrase($this->getLabel())) this call to escape the HTML. So your call to the font is stripped out before it's rendered.
The only way I have been able to get this to work is to place the call to the icon in a .phtml. So in your case, you might just call a template file from the default.xml you are using and call the icons in there. This would be how you would pull a template into the footer container:
<referenceContainer name="footer-container">
<container name="social_footer" label="Social Footer" htmlTag="div" htmlClass="social-footer" before="-">
<block class="Magento\Framework\View\Element\Template" name="social.footer" template="Magento_Theme::html/custom-social.phtml"/>
</container>
</referenceContainer>
And then create a file at app/design/frontend/{{Vendor}}/{{Theme}}/Magento_Theme/templates/html/custom-social.phtml
