FontAwesome tags removed when switching between Source and Wysiwyg
As Vic Nicethemer pointed out - the solution (for me) was to add &amp;nbsp; within the &lt;i&gt; and &lt;/i&gt; like so: &lt;i class="icon-facebook"&gt;&amp;nbsp;&lt;/i&gt;.

CKEDitor removes elements that do not have any internal content...