Facebook puts up the following notice to IE6 users:
Did you know that your browser is out of date?
Microsoft Internet Explorer 6 is no longer supported. To get the best possible experience using our website we recommend that you upgrade to a newer web browser such as: Firefox, Chrome, Safari or Internet Explorer 8. The upgrade is free. If you're using a PC at work you should contact your IT administrator.
IE6 has been on the Facebook chopping block for about a year. See Facebook Doesn't Really Support IE6
What this mean for site owners, web developers, IAs, and the like is that one more nail is being put into the IE6 coffin. I'm not a Microsoft hater but IE6 is a terrible browser and testing for its non-standard idiosyncrasies takes an awful lot of time (money).
Of course, until big corporations do a system wide upgrade IE6 will continue to be a thorn in our side. Large corporation don't care if their employees can't use Facebook. It may even be considered to be a reason to forstall upgrades. :-) Nonetheless when major corporations, such as Facebook, stop supporting IE6 the writing is on the wall. IE6 is going the way of NN4+
As a side note, as far as my sites are concerned, IE6 is about 15% of the total usage. That's too high a percentage to ignore.