Social Bookmarking Icon, is it necessary ?

I’ve seen lots of blog using the social bookmarking icon on all of their blog post / article, especially when viewed as single post. But is it necessary to use those icon ? because i always think that most visitors don't understand what that button actually does. Beside if they were a casual social bookmarking users, they already have their own tools to simplify submitting stories into their favorite social bookmarking network.

Although i don't really care about that small button as long as it's just a text, but most of blog on the internet i saw using small 12x12 / 14x14 / 16x16 image size, and of course if you use them all, it’ll make your visitors uncomfortable especially for those who still using limited connection ( i know that visitor browser will cache those image but it’s not hurt to change it into text, right ? )