Diigo is an excellent source to use. It is great for education for many different reasons. You can use tags on diigo which are like one or two words that are related to the post. You also have a little place to organize your post called the library which is where you can save posts if you aren't done with them, you can put post there for the time being just if you don't want to post something right away maybe.

You can join into groups, maybe with your class, and you can join into more than one group at once which is pretty cool.  When you are in a group you can see what everyone has recently posted and it could be an article on something that your class might currently be learning about and you can comment on their post and you can even like their post. You can post interesting things on what your class is learning about too, and everyone in your group will be able to see it. Well that's all I can say to pretty much sum up diigo for you. Diigo is an excellent source and I recommend you to try it if you already haven't.