There are many means of communicating with each other via the Internet. There are discussion forums, chatting sites (IM), web conferencing, audio and video phone calls, and of course email.

Blackboard, Moodle and Angel are all Course Management Tools and there are numerous built-in features for students and teachers to safely communicate.

Skype http://skype.com talk and chat online for free
Google Talk http://google.com talk and chat online for free
Twitter http://twitter.com stay connected with quick and frequent answers
Tokbox http://tokbox.com send video emails to your friends
Vyew http://vyew.com/site/ live web conferencing
Yugma http://yugma.com/ live web conferencing