There are hundreds of different social bookmarking sites out there, and they all work to some degree to either improve your site's Google rating, or drive traffic to the site.

There are two main ways that a social bookmarking site can improve your Google rating. One is by letting link weight flow, (which helps to boost your Google rating), and I will explain what that means in a second, and the other is by driving traffic to your site directly from the social bookmarking site.

Link Weight

Link weight is an algorithm that Google use to determine the amount of "points" to give a site, based on the quality, and relevance of the sites that lead to it. Each site has a certain amount of link juice that it can pass on to another. There are different algorithms to determine whether the link text used to make the link is relevant to the site it's linking to, and the amount of link juice transferred is divided by the amount of links from that page.

Also, a site can sometimes have no follow links. This means that no link weight at all flows from the page, to the site it links to. There are quite a few social bookmarking sites like Twitter which have no follow links, but Google also has a separate algorithm to calculate the popularity of that link on a site like Twitter.

Basically what this means is that Twitter will work, if your site is tweeted by lots of different people. Facebook is also no follow, but if many people "like" a page, recommend it, etc, Google also recognizes these social signs to some degree.

Other sites like Digg have do follow links, but in the same way, it is better if many people "digg" a site, rather than just one person. Stumble Upon is no follow, but this site has the benefit of lots of traffic if used correctly, and the best way to use it, like all the others, is with a group of people. If you are writing on Factoidz, check out the buzz it up page, to swap social links with the other writers here.

Traffic

Sometimes social bookmarking sites are most effective because of the traffic they bring to your site. For a social bookmarking link to bring traffic to your site, then either the links need to be found in a Google search, or the links need to be found by other users on that site, and the only way to make a link popular on a social bookmarking site is by getting lots of users to like it, digg it, tweet it, as well.

If your site is dugg many times, it appears in the feed of all your followers, and it may even get popular enough to get on the top news section, which is seen by hundreds of thousands of users on the site.

If your site is stumbled many times, (or liked on Stumble Upon), you will appear more often in the random surfing, or "stumbling" that some users do. Obviously, if your site is tweeted on many different accounts, so long as they have lots of followers who are looking at the tweets, you will get lots of traffic from that too.

Social bookmarking works a little bit in link weight to improve the Google rating of your site, and a little bit in traffic to improve your Google rating, and traffic is another algorithm that Google finds important.

The real key, is that to make the most out of social bookmarking sites, it really requires a team, and if you can't find a team of users to bookmark each other's sites, you can pay for other people to do some social bookmarking for you, on sites like Fiverr.com, Webeserve, Microworkers, etc.

It is only really worth paying for social bookmarking if you can't find a team, and have a big website that will make you a lot of money if it is on the first page of a Google search. There are many aspects to SEO, (Search Engine Optimization), and this is just one of them.