That's because you didn't put your url there.
href="{url/link}"
href="{url/link}"
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#quicknav a.google {
background: url(../images/theme/google.png) no-repeat !important;
background-position: 0 0 !important;
}
if(!empty($settings['twitter_username']))
echo '
<li><a class="social_icon twitter" href="http://twitter.com/', $settings['twitter_username'] , '" target="_blank"></a></li>';
echo '
<li><a class="social_icon google" href="http://" target="_blank"></a></li>';
if(!empty($settings['twitter_username']))
echo '
<li><a class="social_icon twitter" href="http://twitter.com/', $settings['twitter_username'] , '" target="_blank"></a></li>';