nice post! I'm curious: are you using caching?
just a side advice: don't reinvent the wheel: sc.social.like has been around since Plone 3 and it's still the de-facto standard to include social media metadata and widgets in your site.
your Twitter Card implementation can still be enhanced if you remove all the redundant metadata and just let Twitter use the one from Facebook's Open Graph (we're currently working on that).
also, note that the canonical URL as defined by Facebook has some unintended consequences when you move content around; we tried to address that in latest release, but it still has some bugs (#114, #115) and limitations (#119).
wait for release 2.10.1 later this week if you want to give a try.