Been using this since 2010 and it gets better all the time.
This isn't really a pure teaching tool but just a great classroom social network-type of web site that lets you manage materials for classes and clubs. Much like Weebly or some district-provided teacher web sites, it's a class calendar/file upload center/teacher and student note depository but the ease of use is what makes Edmodo stand alone. With great security and parental code usage, it's the system I've found works best for students, teachers and parents. Highly recommended.
How I Use It
This site is used each semester and students are required to set up accounts and are responsible for any information I post on their class pages. Basically, I upload files to each class page either before or after class (sometimes both) so the students access it during the class period or later at home to review materials we covered in school. Since the material is archived, it can also be accessed by those who were absent to see what they missed. Since I teach several preps, my usage may vary depending on the class but Edmodo is flexible enough for me to use it differently for each class. It includes huge amounts of storage and fast server access and it's all free.
Teachers should realize this tool is not a stand-alone lesson delivery system. It's going to be used most frequently in conjunction with a more traditional lesson form or with newer apps or technology. So I'll still have a Power Point or app-driven lesson or even textbook-related work and post the assignment initially on Edmodo and have students go to their Edmodo class page to see the directions for that lesson. Or if we're exploring a web site together, the link to that page will be on Edmodo and it's much easier to have students start with a page already bookmarked as opposed to having to let them struggle to find where we're supposed to be. But it also means I can deliver the same material easily to a larger group of students or those who are absent. And with built-in assessment tools, Edmodo is a good way to produce exit-ticket type of activities. Another frequent way I use Edmodo is to post a video link and design a five or ten-question quiz on that material. This flipped classroom type of lesson works very well on Edmodo since I can see how many of my students have visited that page.