The slight problem with that and even with the implementation on LampCMS, is that for each social network you want to share, you have to complete the auth info at submit time.
I think the sharing could be made seamless by providing a facility for users to pre-fill all that info on their profile page, and help them indicate which networks to auto-share their posts or comments.
On LampCMS, for example, this is what my profile page looks like:
Like I wrote above, it would be nice if I could provide my login details before hand and be able to indicate which social platform I want Discourse to auto-share my posts or responses to posts, provided there are no show-stopper security issues that come into play.
On Feedburner, for example, there is a social feature that auto-shares my posts to Twitter. So I’ve never had to log in and post directly to my Twitter account, only via Feedburner.