What is the function of a social bookmarking site?

I use RedGage as a way to build a community familiar with my work. I like the sense of community there and it does help to promote my writing at other sites.

I use facebook, where I can make friends, offer a form of business that I do even to upload unlimited images. there also I can keep in touch with old friends and even distant relatives

Some of them are good, some of them are not. I have had good results with Digg and Stumbleupon. So what I suggest you do, is to visit those sites (or some other sites that you have heard of) to see what goes on there. It is the best way to get these questions answered. I do think they can work for you, they have for me, but only the two I mentioned, the other ones I tried were pretty much useless. So choose the sites carefully!

yes you are right i have also read everywhere about this sites to increase traffic to our blog or website but i dont think so that it works as they promise i have some friend who own their blog and they have place their bookmark at such sites but they usually dont see any rise in traffic or page view from such sites. I think they all are worthless.

Well it gets those websites more traffic and since it's based on user's preferences,stumble upon is at least,it sorta has a higher rate of people who will subscribe to that page or will keep visiting it and perhaps tell friends about it.