The effect is zero! They do nothing for your website when directly pointed at it. Great for diversity zero for quality and moving up the SERPs. I am even of the opinion that social bookmarks could sometimes harm your website. That bad!!!
You asked for the effects on a website… The effect is Zero… Social bookmarks have their uses. But I personally have never seen them impact the SERPs for a website much. I use social bookmarks. I simply do not use them on my websites.