Are ‘share this’ buttons really effective?

On a lot of websites (especially blogs), we find 'share it' buttons like this --> .

Personally, I never used one. If I want to share something online, I copy the link and manually share it through SNS or IM. Other people claim that they use it, though.

Overall, are 'share this' buttons really worth putting on websites? Yes, they usually consume only a small amount of screen space and bandwidth, but is it worth it? Aren't they just adding to visual noise?