First is information overload. It’s very common to see blogs with a whole row of social bookmark tags. I think that these are going the way that banner ads have over the last few years; site visitors have a blind spot for them.
Second, if someone wants to bookmark your site with one of these other sites, they can do it anyway; they don’t need your insistent icon asking them to do so.
Last, according to Digg itself, 94% of Digg users are male. 88% of Digg users are in the 18-39 year age range, and 52% of them are "IT professionals, developers, or engineers." That’s a very narrow demographic. Unless you have a blog that is targeted at this demographic, none of the ensuing traffic you might get will do much good anyway. It’s unfocussed, untargeted, and will not convert very well.