If you access the Web :: Top Websites report which can be found under Plugins->Web->Top Websites
Fill the Website Name filter field with a selection of Social networking sites for example
facebook|bebo|myspace|youtube ie website names separated by the | symbol.
Click View and the accesses to your specified sites will be displayed.
Once you are happy with the filter you can save the report for future use by selecting More Actions -> Save Report. This will allow you save this report as a Custom Report rather than having to regenerate the filter.