Important: This is a proof of concept and should not be used in a production environment in its current form! Important: I did not take all the hassle to make it work with MSIE. It works best in Firefox.
This is a moodle block that uses the clever concepts outlined by Mike Cardwells Blog Article to check wether the user is logged in into certain famous websites like Facebook, Digg, Gmail or Twitter.
Update Jan 27, 2011: Facebook has added a Captcha, which makes this concrete implementation useless for detecting facebook-logins at the moment.
Download the file and unzip it into your Moodle blocks folder. Log into your Moodle as admin and select Modules → Blocks → Manage Blocks. Here you can activate and deactivate the block.
Once it is activated, you can add this block to any course you like.
You can use the PHP library outside of the Moodle block to detect if your users are logged in. See the the demo-site below.