I can't personally speak to any research that shows usage rates of
specific links, but I do believe social links can be very effective.
As more users adopt aspects of the social net into their lives,
"email this item of interest to a friend" becomes less valuable
then "broadcast this item of interest to my network." Adding
sharing links to pages/content can be an effective way of spreading
content, building contextual relevance, and increasing natural search
Segmentation and other user research should give you an idea which
networks your users may have adopted, giving you a rationale by which
to prioritize. There are also services like AddThis (www.addthis.com)
that aggregate the many popular sharing tools into one button that
avoids a lot of clutter. As an added benefit, they include analytics
so that you can get some real usage data back.