According to Microsoft’s Australian chief security adviser, Stuart Strathdee, companies are avoiding upgrading their internet browsers from IE6 because Facebook, among other things, doesn’t render properly within the browser. While the companies could theoretically use firewalls, or other security measures to prevent Facebook usage, many companies simply choose not to upgrade browsers instead.
Web designers and developers cringe at the idea of supporting IE6 which is why many companies, including Facebook, have decided to stop supporting the browser all together. Often times it was the source of many front-end developers pain, as they added CSS hacks just to support a browser which many would have hoped to be deprecated years earlier.
However there are still stragglers who are set in their old ways and have no intention of upgrading their browser, since IE6 still works for their email, Google, and most other things that they use on a daily basis (aside from Facebook). While we stopped using IE6 many years ago, it appears that some companies have finally found a benefit to using a service which is considered ancient in technology years.