Probably some overlap between the 3 but I would like some intelligent
definitions of what each of these might be with some examples of
services in each category.

THANK YOU!

Social Bookmarks – storing pages and links of sites you like with keywords or tags describing them. Other users of the social bookmark sites have the same keywords and the results are displayed when searched for.
Examples: Delicious, Spurl, Jumptags.

Social Networking – users have profiles which include geographical location, employment, schooling, and interests. Other users can search by these parameters and make friends/connections.
Examples: Facebook, MySpace, LinkedIn

Social Media – users can submit and share stories. They can also vote on stories and "push" the story to the top of the list for better exposure.
Examples: Digg, Slashdot, Fark

Hope this helps,