Personally, I think social media and academic publishing timescales, rewards and social conventions don't match very well. Social networking feels transient and impersonal. I would like to take some time to form my opinion about a paper, not jump in with a post. Maybe some comment box would be ok to write a couple of nice things about a paper, but it doesn't feel the place to write harsh criticism or have complex discussion where things could be misunderstood. Rather than write in the public record, if I think the paper has a deep flaw I would prefer to contact the authors first. This can be followed up by discussions in your own papers. Others may have different opinions, of course.