I manage a few decently sized blogs for authors and bloggers. From all the checking I've done for these buttons, only Email To A Friend, Facebook, Twitter, and G+ are worth doing.
The others might get 1-10 clicks vs the aforementioned few getting thousands of clicks (or hundreds for G+).
I've gone to just putting Facebook/Twitter/G+/Email, as minimally as possible.
Depends on the crowd. I wrote a blog post called Hackers and Engineers (the target audience is obviously the same people who read Hackers and Painters) [0]. From what I can see, lots of entries, lots of clicks on the links of the blog post, none on the share/tweet/facebook/g+ buttons.
By contrast, I see people clicking on the tweet/share/facebook button for the blog entry where I compared memes to speaking in metaphor ala Darmok and Jalad at Tanagra [1]. They don't click on the links (except to the book Wisdom Sits in Places)
Different crowds of people react differently to those share buttons, I guess. The HN crowd doesn't really click that much.
Yes people use them. They can drive a decent amount of traffic too if they have some alluring and easily digestible content such as the common "list of top 10 bla bla" articles. It is also suspected that Google weighs shared links highly for ranking so just a few shares can give a substantial boost to a page's seo value. Though no one knows for sure how it works.
Using Ghostery here... my initial perception of a site is always based partially on how many entries are blocked on load. I've seen pages with 20+ calls to outside sharing services; at that point, a little extra cynicism kicks in.
The others might get 1-10 clicks vs the aforementioned few getting thousands of clicks (or hundreds for G+).
I've gone to just putting Facebook/Twitter/G+/Email, as minimally as possible.