"If what you are getting online is for free, you are not the customer, you are the product."
It isn't the first time this sentiment has been expressed. Lifehacker quotes a Metafilter user from a couple years ago (blue_beetle) saying "if you're not paying for something, you're not the customer; you're the product being sold."
There are other iterations of this same thought with various sources - but the salient point for us would be that it never loses accuracy: Each of these speakers was referring to one or some of the abundance of free online tools now available - likely the very same tools you're thinking of using in your class.
Even by requiring a student to sign up for a service, or requiring that a student connect to certain other people using that service, a little bit more of that student's information is connected for data-mining. If you're thinking of using a web-based log-in required tool, such as Facebook or Twitter, in your class, here are just a few of the questions you should ask yourself:
- What learning objective am I meeting by using Web 2.0 / social networking tools?
- Is my subject one where students will want to be connected to each other outside the duration of the course?
- Am I comfortable using the technologies I'm suggesting?
The third one may be the most critical for student privacy - if as the instructor you are uncertain as to how to keep posts private, how your technology choice may be influenced by FERPA regulations, or about who owns the words and pictures shared on the technology after they're posted, then perhaps there is another, better, way to achieve your course goals.