Just to clarify for people who are not 100% code savvy, change the abcd to the field number as stated in the "User Profile Field Manager" e.g. field6, field7, field8, field9.
So if your twitter profile was say field 6 then it should be changed to the example below
Code:
<vb:if condition="$post['field6']">
<a href="http://twitter.com/{vb:raw post.field6}" alt="Twitter" TARGET="_blank"><img src="images/site_icons/socialpopup/twitter.png" /></a>
</vb:if>
Hope this helps