do_shortcode for twitter doesn’t work well

You are linking to a support page for wordpress**.com**. While the basic Core WordPress is the same, that is a hosting service and it runs some code that is not part of Core. What you see there may not work on a self-hosted WordPress install. What should work, however, is this:

do_shortcode('
Really cool to read through and find so much awesomeness added to WordPress 3.6 while I was gone. I should take three weeks off more often.
— Andrew Nacin (@nacin) April 3, 2013
');