Diigo is a social bookmarking site which allows users to save links to various websites. This can allow a teachers to set up a list of sites for students to use on a certain project or given a certain focus. When you have a large number of sites, it is easier to use Diigo in many instances because of their use of tags. Students are much more likely to find useful information if they are to search for "Gettysburg" vs "The Civil War." Being able to filter is a huge advantage in this context.
Pinterest allows for a much more visual representation of the material in which you are searching. I think that if a student were to pull up both a Pinterest page and a Diigo page containing the same information, more students would use the Pinterest page due to its visual nature. I think that by creating a medium where students feel that what they're doing is less like work and more like a game, they are more likely to become engaged in the material. Engaged students are more likely to spend more time with the material and, in turn, learn more about the topic.
Other differences include: being able to comment directly on Pinterest posts while this is not necessarily possible on all Diigo posts, the ability to add your commentary directly to your posts on Diigo (via the description you create), the ability to private message those you are following on Diigo, and the ability to automatically find articles related to whatever you have searched on Pinterest. Both sites allow me to follow others, as well as be followed back.
Other differences include: being able to comment directly on Pinterest posts while this is not necessarily possible on all Diigo posts, the ability to add your commentary directly to your posts on Diigo (via the description you create), the ability to private message those you are following on Diigo, and the ability to automatically find articles related to whatever you have searched on Pinterest. Both sites allow me to follow others, as well as be followed back.