Tracking user engagement on Facebook fan pages with Google Analytics

Talk on the Google Analytics Authorized Consultants board is that this solution produces unreliable numbers.

Here's the analysis posted from Mike Plummer, another web analyst:

Code does not read utmz cookie from firefox but does from other browsers. Code writes incorrect referrer cookie as the current page URL.. since the code cannot see the true referring URL as it’s being instantiated from an img tag, so the referring URL will always be the current page being viewed. This means the traffic sources reports are not reliable and definitely means you should not use your main domain’s cookies i.e. company.com with this script as you’ll pollute your man GA profile cookie data with incorrect or partial data. You will get a reliable count of unique visitors and visits so far as I can tell. I would not advise using this code to write utm cookies on your live domain though anyway, just in case it’s corrupting the utma cookie in some way I can’t detect. GA also throws away visits with invalid utmz cookies, so firefox visits will be incorrect. The pageview metrics are going to be off too. Every browser I tested caches the tracking pixel since there is no cache defeat mechanism.