Oftentimes people point to research conducted in a university context to ask why teachers aren't implementing social media services into their teaching practice. The simple answer is schools are different to universities, our clientele is different. The clientele of a university is it's students - the clientele of a school is society in general and parents in particular. Before we try and convince teachers to grasp change in education, it is always necessary to convince parents that it is safe to do so.
Ten years ago I was teaching students how to use email using Hotmail. Within a very short time Hotmail and other web based email was blocked by the school system I work in, and it took a number of years before an email service was provided to each student. Why was email blocked and then released under strict control? Because of the fear of students sending inappropriate messages to each other. YouTube is blocked. Why? Same reason, only its video not messages.
No doubt, in a few years time all manner of social media services will be available to students under strict, filtered conditions. In the meantime teachers will be unable to fully implement Personal Learning Networks for their students.