What is the best way to use online mathematics forums, such as mathematics.stackexchange.com or Khan Academy, in mathematics instruction?
I regularly encourage my undergraduate students to participate on mathematics.stackexchange.com (and my graduate students are all on mathoverflow.net). I tell them to post any questions they might have about their homework or other topics we've discussed in class, and I am confident that they can get high-quality mathematical guidance by doing so. Many of my students have learned a lot this way. I give extra credit to any student who can accumulate sufficient reputation points, and I am significantly more impressed by students who do by answering questions rather than merely asking them. Some of my students have gotten hooked and are now active participants.
What kind of experiences do other instructors have using math.stackexchange as part of their instructional experience? Has it been successful? How have you encouraged your students to take part? Do you give credit for earning reputation points? Do you worry about the students conspiring to vote each other up? Do you discuss explicitly in class the proper way to ask a mathematics question in such a forum? What are the main stumbling issues and success stories?
For my part, it has mostly been really great, with many students getting help with issues with which they were struggling. Some of my undergraduate students, however, are initially stung by a few downvotes or critical remarks on math.SE as they ask their first few questions, when they do not yet know the proper way to ask. Some are put off by this and are too timid to participate more fully, but I always encourage them to try to preservere. Others carry on and end up asking and answering a lot of math questions there, and I believe they really learn a lot that way.
I have less experience with Khan Academy with my students, since the level is much lower, but I know from experience with my elementary school children that the Khan Academy is an excellent way to learn math. Please post an answer if you have experience using this or some other interactive online mathematics forum in your classes.