Both Flickr and Diigo are similar, except that Flickr is for photos and Diigo is for information. (How is that for over simplification? LOL) I noticed the similarities in the ease of use (both practically do themselves), the ease in sharing the links/photos, and the ease of joining a group. You can have a private photo stream (diigo bookmark list) and a public one.
I enjoyed using them, though when it comes to putting pictures on Flickr, I don’t think I would use it much. Any pictures I upload are usually to my Facebook account, though Flickr is MUCH faster. I like Diigo, because research can be a tiresome, irritating endeavor. Nothing is worse then finding the perfect site for something I am researching and then closing it by accident and not being able to find it again or trying to find a quote for a paper and not being able to find the author. Having the tool right there reminds me to bookmark the page I am on. It also lets me tag the bookmark with a reminder.
I will be keeping both of them though, at least until I am out of school. I like searching for pics on Flickr, even if I am not going to post many pics there. I like Diigo for much the same reason-I like searching what other people find interesting!