Far better to say something like “we can’t tell what browser you’re using. See (handy link to possible reasons and cures). We don’t want you to miss out on Facebook’s features.” But that wouldn’t be all hip or ironic or postmodern.

I do think it’s obnoxious to completely block unsupported browsers though. I just saw a CSS/Javascript trick to make sites unusable in IE6 because developers want to force everyone to upgrade to IE7 (or whatever they support). I think it would be better customer service to simply allow the site to show and get as much as they can. If stuff doesn’t line up quite right or not all the functionality is there, well, then people can deal. They’ll complain, you can recommend an upgrade, etc. Then you don’t get the issues like this one in Facebook where people get blocked based on false alarms.