Edmodo just rolled out a new release last night and in it they implemented student profiles and badges [link to blog post]!
Um... what are badges? They're kind of like Boy Scout or Girl Scout merit badges. Online or in Edmodo, they're kind of like customized stickers that you get to create (whatever picture you like) and establish the criteria for (totally your call and discretion). When a student demonstrates proficiency or meets your criteria, you can award him/her the badge and it appears in their profile! The only people who get to see a student's profile and badges are that student, his/her teacher, and other students enrolled in the group (a.k.a. class).
I've already created two badges for my science students: "Data Cruncher" and "Inventor". I'm now working on criteria for "Scientist" levels 1-3 for designing different types of experiments and participating in a science fair (that doesn't exist yet... but might soon).
This is very similar to how the Khan Academy rewards students with badges.
I admit that I am not certain that I philosophically agree with these rewards and motivators. But I've seen some pretty apathetic students work for over an hour on basic math fact drills on the Khan Academy just so that they could get that badge!
Alfie Kohn and "Punishment by Reward" definitely have a point. But based on what I've seen amongst some of my own students I have to reserve judgement and even admit excitement!