Less than .5% of web site traffic was driven by social networking sites based on analysis of 6 UK charity web sites for the period 1st Oct 2009 to 28th Feb 2010. The sites examined had monthly Visits between 20k and 800k.

Most popular Social Sites

Social Sites such as Facebook and Twitter are now major sources of traffic for many web sites and for some web sites rival Google in their ability to drive traffic.

However for these UK charities the social  networking sites are not big drivers of traffic. The analysis showed that Facebook and Twitter accounted for over 95% of visits from the social sites, although Stumbleupon had some success. While the data says that Twitter brought about 33% of the visits that Facebook brought in reality Twitter probably brought more as many believe that Google Analytics undercounts Twitter visits by a factor of 5 or more.
Social Networking sites have an important role to play in reaching specific audiences but for now it is best to tag each tweet and check the stats for the specific landing page to really measure the impact on visits.