Oh, I glossed over that this is for custom links rather than sharing.
You could probably add an Instagram button like this:
	
	
	
		HTML:
	
	
		<a class="shareButtons-button shareButtons-button--brand" href="...">
    <xf:fa icon="fab fa-instagram" />
    <span>Instagram</span>
</a>
	 
 
...but you'd have to add custom styling for the hover effect.