"There is no reason sharing actions, etc., should not be handled by the user agent."
Wait, why stop there? Aren't there all kinds of actions that should be handled by the user-agent? There are N verbs that we could get excited about wiring up, across M potential service providers. Woo hoo!
Of course, once the enthusiasm for the shiny wears off, we don't really want to have to experience the web this way. Do we?
(I would also like to see browsers more closely support e.g. the big JS libraries like jQuery (UI) et al by having a local copy you can opt in to using instead of having the user download them yet again, but that's a separate dream.)