I just recently joined twitter since I started my newest job. We use twitter as a promo tool. Which can also be used inside the classroom. Teachers can have a twitter account for the classroom. The students whom have a twitter can follow the class and see assignments and reminders about due dates and more!
I am not a fan of facebook. In the past year facebook has changed in my opinion. It went from a tool to help communication to a tool that is dangerous for some to use. Kids can be bullied on facebook and twitter. Facebook has become more of a website to record all your life, an autobiography.
Both of these sites allows for faster and easier communication with students parents and faculty for schools. Even though these sites can be used as a tool for education they still must be used with caution.