That's indeed a good point. However, regarding the Author link - the issue is having it twice in the same post.
First, at the beginning of the post (as name of the author). And then, at the end of the post in Fancier Author Box.
I just felt having two dofollow Author links on the same page pointing to the same URL (and then, this pattern repeating itself on all the guest posts) may not be good.
As for a nofollow for Social Media links, I have seen most of the well established sites following this practice. Hence the request.
It will be good to know what you think as the best practice on both these issues.