There is overlap between most linked, shared, and commented so some of the above would fit in several categories but it’s surprisingly weak. Posts with a lot of comments, for example, often do not draw lots of links. -You would logically think that an article that engaged readers sufficiently to result in a comment, a share or a link would generate a relatively uniform response across all three forms of engagement. The amount of time necessary to comment, share or link is about the same. Why the disparity?
Stepping back and thinking it through though, I see that Tabarrok's comment is less surprising on reflection. I link numerous articles, I only share a few and I rarely comment. Why the differences? I link to an article in Thingfinder in order to discuss some core idea or I link to the article at Commonplace of a Magpie in order to have the article on file because I think I will want to refer to it at some point in the future.
I share with family members and close colleagues when I think the article contains information they will find amusing or useful and might not otherwise know about.
I comment on articles where some basic information is being overlooked and causing the conversation to veer away from reality.
The time requirement is about the same, regardless of mode of engagement. However, there is high return to me in linking, only some in sharing and virtually no return in commenting. Is that profile common? I don't know but perhaps, and if so, that would explain the differences in the form of engagement alluded to by Tabarrok.