Diigo was actually the first bookmarking tool that I started using. I don't think Pinterest or Scoop.it functioned then. Maybe they did, but I didn't know about them.
Diigo allows users to bookmark a webpage, to highlight it or to attach sticky notes. All bookmarks, highlights or notes can be shared with groups in Diigo or they can be kept private. To make finding links easier, tags can be used. The tool also allows users to upload pictures and save them. There is also 'Read Later' option which one can use when they have found an interesting article to read but do not have the time for it. Not to forget what they wanted to read later, they can bookmark the article in the 'Read Later' section and then find it easily.
Diigo has an optional bookmarklet which allows to bookmark any website from the browser without opening up the Diigo page. However, Scoop.it and Pinterest also have these bookmarklets so this is not something specific to Diigo only. Although I don't use Diigo very often nowadays because of Pinterest, I still have the account and bookmark important articles or save links to my Diigo library if they cannot be Pinned.
Pinterest is also a bookmarking tool, however it bookmarks pages on which images can be found. If there are no images, a message comes up saying that the page cannot be pinned because no pinnable images were found. This is a bit disappointing really but still I think Pinterest is a great tool. I mainly use it for videos that I want to return to. I also have an educational board on Pinterest where I pin various infographics and articles related to teaching and learning.
It is easy to create separate boards and then pin links to them according to their topic. The same can be done with Scoop.it. In general I think Scoop.it and Pinterest are more or less similar. The biggest difference between these two bookmarking tools is that the scoops on Scoop.it come up with snippets of the text which can be read through but on Pinterest only images can be seen.
Commenting, sharing, embedding, re-pinning/re-scooping are all available with both tools.
Another difference between these two tools is that on Scoop.it you can manage sources by adding keywords of topics of interest to the dashboard and with one click Scoop.it searches the web for blogs, articles and more that match your interests. The links are then suggested for scooping and then can be scooped, removed or discarded. Keywords can be easily edited at any time. The free account on Scoop.it allows 5 topics.
Overall, all the three tools are great and easy to use. It is just a matter of personal choice which one to use and for what reason.