1. Posting a comment to a video on YouTube. It's not like you ever go back to check on the response, is it? You just post into the void, and move on.
2. Rating other people's comments. Since they'll never see them (see 1. above) what's the point?
And yet we do it. The very idea of posting a reply to a comment made a year ago has an irresistable appeal to me. Feel free to give your own views in comments, in about 2012.