Inspired by the How To Share Google Reader Stories to Google Plus article from Read Write Web, I just figured out how to do the same thing for sending a Google Reader story to the Diigo bookmarking service.
Head on over to Google Reader, click on the gear icon in the upper-right corner and select Reader Settings.
Once in settings click Send To. A the bottom of that page click Create a custom link and fill in the form with the following values:
Icon URL: http://www.diigo.com/favicon.ico
Click the save button when done.
Now you’ll have Diigo as a share option in Google Reader. When clicked it will take you to the Add a link page in Diigo with the title and URL already filled in. Just add a description and tags as needed and save. (If you’re not already logged into your Diigo account you’ll be asked to do so.)