I would recommend reading this article by Oliver Reichenstein of Information Architects. He discusses (in a two part article) the issues that arise with using these social share buttons.
They are very interesting and thought provoking...although I am not saying I 100% agree, but it is worth the read(s). The first article is Oliver's take on the issue and the second article is rebuttals from numerous sources.
A couple quotes from the articles:
"We find content through Facebook, Twitter, Google+, Pinterest etc., not the other way around."
"If readers are too lazy to copy and paste the URL, and write a few words about your content, then it is not because you lack these magical buttons."
"In any case, what we suggested is not cutting social media off, but integrating it more selectively and consciously. To further careful social media interaction and conscious debate. This is how we think social media works."