Twitter and Facebook have almost doubled the amount of traffic they sent to 6 UK charity web sites over the last 6 months. Visits identified as coming from all social media sources, have grown faster than the overall growth in web site visits while Facebook and Twitter are sending a greater share of the overall social media traffic. However for the sites that I reviewed for this survey social media is not a big source of visitor traffic. In the last 6 months approximately 1% of their total web traffic has come via social media compared to .5% in the previous 6 months.

Facebook and Twitter are most popular with UK charity visitors.
The share of traffic from social media sources varies among the surveyed sites from a (very) high 6% to less than .5%. Twitter and Facebook are the largest contributors to social media visits and are the fastest growing of the social media sources. Comparing the last 6 months with the previous 6 months Facebook sent 65% more Visitors while Twitter sent 80%. If you are comparing the absolute number of visits remember that Twitter is reckoned to be under counted in Google Analytics by a factor of 4 or 5.
This analysis is similar to the social media analysis I did back in March 2010. This time I included Visits from over 34 social social media sources and ensured that the analysis was comparable across all 6 sites for both periods. Three of the sites had visitors in the 10/20k per month range with three more having over 100k visits per month.
The data comes from Google Analytics using the Advanced Segments capability. You can do this test on your own web site by using Google Analytics Advanced Segments. You can easily create you own version of the Advanced Segment or drop me a line and I will send you the one I used with its specified social media sites. Once you receive it simply log into your GA account and paste the url into your browser. Voila ! you can check how your site compares to the ones I did.

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You can track how many visitors a page on your site receives as a result of your Tweets by Tagging any Links that you put into your Tweets. Bit.ly, currently the most popular URL shortner, can be used with Google link tagging. Simply add tagging parameters such as Source and Medium to the end of the url, shorten in Bit.ly and post to your Tweet. Now when you look in Google Analytics > Traffic Sources you will see a separate Medium ` Twitter` and provided you have tagged your links they will appear listed for each Tweet you have sent out. Now you know how many visits your links have brought to your site.
Incidentally if your site uses the AddtoAny bookmarking service ( as this site does) then you can see full details of what pages have been bookmarked by looking in GA >Content > Event Tracking.
Combining the use of Advanced Segments, Link Tagging and Event Tracking will give you a good idea of how your Facebook and Twitter activity are affecting your site.