I found this fascinating quote today:
A few weeks ago, Facebook unveiled the ability to tag, or electronically connect, friends and other Facebook users in your status updates, notes and such using the “@” sign before their name. As you start typing the name, the list of friends comes up, just like they would if you were addressing an email to them, and their name then magically becomes hyperlinked to their profile page. The same works for group or pages you’re a member or fan of.socialmediaexplorer.com, Facebook Tagging For Business, Sep 2009
You should read the whole article.