It'd probably be monopolistic if they blocked IE8, and maybe IE7. But I think IE6 is legitimately old enough, with enough known problems, that it would be justifiable if it ever came up in court.
It' doesn't take much for Google's web search to support an outdated browser since it uses minimal javascript, so Google will only lose out by blocking IE6.
I wonder if facebook was paid by adobe when facebook videos began requiring Flash 10. That's the only reason I upgraded flash.
Having developed Flash plugins and applications in the past, you need at least Flash player 9+ to do anything remotely powerful.