Code:
<vb:if condition="$post['fieldX']"><a href="http://facebook.com/$post[fieldX]"><img src="http://facebook.com/favicon.ico" /></a></vb:if>
If the user inputs into fieldX the value 15253535, then it will take them to
http://facebook.com/15253535 when the facebook link is clicked.
You may want to save the facebook icon to your server and do a relative link <img src="/images/facebookicon.gif" />