Help please - some weird PHP

I have a site that displays the facebook page of a user within a profile page.

When I test the site by creating a test user everything is fine, but when users sign up the site put in full domain name before the the facebook url

my test site - http://facebook.com/myfanpage
user signs - http://domain.com/foler/http://facebook.com/myfanpage



This is the code.
$fb = get_post_meta($mypid,'facebook',true);




if(!empty($fb))
{
echo '<a href="'.get_post_meta($mypid,'facebook',true).'">Facebook</a>';
echo '<br/>';
$mp++;
}

Any ideas?

It seems to be ok if I echo the it out - but when it becomes a link it adds full domain path.