Twitter and RSS feed icon in the footer. Adding other social media

In the footer a text widget has been added and inside it this is placed:

<div id=”avia_socialcount-3″ class=”widget clearfix avia_socialcount”>

<span class=”social_widget_icon” data-av_iconfont=”entypo-fontello” data-av_icon=”” aria-hidden=”true”></span>
<strong class=”asc_count”>Volg mij
<span>op Twitter</span>


<span class=”social_widget_icon” data-av_iconfont=”entypo-fontello” data-av_icon=”” aria-hidden=”true”></span>
<strong class=”asc_count”>Subscribe
<span>op mijn RSS Feed</span>

<span class=”seperator extralight-border”></span>
</div>

I want to change the RSS icon to a Facebook icon and add LinkedIn and Youtube. However inside data-av_icon=” ” there is a little symbol. I can’t seem to find what it is, where it came from or how to find symbols for Facebook, LinkedIn and Youtube.