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 talk and chat online for free
Google Talk talk and chat online for free
Twitter stay connected with quick and frequent answers
Tokbox send video emails to your friends
Vyew live web conferencing
Yugma live web conferencing