The risk is that publishers might then start opposing the publication of manuscripts that have been shared as preprints on arXiv, if they start perceiving it as a competitor and not a supplement to their "service". But I concur, arXiv with social media features would be nice.

I like this idea.

But it should be a separate entity for robustness's sake.

Let's make it! We'll even hold weekly live video/audio discussions on each paper for it's lifetime.

Let the errant and emerging thoughts of researchers accompany the original research so we can better iterate and grow. In the open.