When someone I follow on Google+ posts something I like, but not necessarily enough to pass along to other people or to comment on, I give that post a +1. It's my way of agreeing in a quick, tip of the hat type of way. If a post inspires me, or to paraphase C+C Music Factory, makes me say hmmmm, I'll comment on the posting. Think of it as a +1 with interest. Now, if a post really engages me, to the point where I find it so interesting or like it enough to give it the stamp of approval of my name, I'll share the post.
The whole structure breaks down to how much a post or an author interests me and how much I want to share that interest with the others who follow me.
Twitter is similar but a little more limiting as to when I can comment on things due to the 140 character limit. In that world, I follow the favorite - retweet - reply model. If I like a tweet and I want the author to know I like it as well, I'll favorite the tweet. If I want to make a point of my followers seeing a tweet, I'll retweet, and if I want to add my two cents, I'll reply. The key for me on Twitter is not so much the sharing of the original tweet as it is interaction with the tweet's author.
The advent of social media has taken our ability to share the content that affects us with a larger audience than ever before. As long as we adhere to these kinds of standards and don't do dastardly things such as stealing content and taking claim for work that isn't ours then the sharing nature of social media benefits us all; personally and professionally.
P.S. - For that mom out there who was confused about my sharing her daughter's blog post...it was done so more could enjoy her post. Every writer needs a vocal, sharing audience. :)