Honestly, when you are dealing with a website where page views are your main source of revenue, every little bit helps. Like you said, the button does not take up much space or bandwidth, so it doesn't really detract from the user experience much, if at all.
Our research on tasteofhome.com with regards to these links showed that it was about 1% of the users that ever clicked on them (so the 90/9/1 rule is probably accurate here, as mentioned above), but they did click on them. This means that they did add to traffic for the site in some way, which does add to the site's revenue. As long as you aren't manually adding them and you can find a visible place to put the button that doesn't detract from the style and overall look of the page, then I would say that you definitely go ahead and use them.
Also remember that even if very few people use the button to share your content, they ARE adding another external link to your page, which also helps with SEO.