For class announcements, I use both ANGEL e-mail and a course blog (from the Blogs at Penn State) for announcements and Penn State e-mail for one-on-one communication. The blog was useful because it kept an archive of what was posted in a central location which students could reference later (a function duplicated in the ANGEL Announcements tool).
In the past I have used synchronous chat tools like AOL Instant Messenger. If I do this, I like to copy and paste the session into a separate text file so I have a record of what was said.
I have used both ANGEL discussion boards and individual student blogs and ANGEL Discussion Forums as platforms for asynchronous discussion. I like asynchronous discussion because it allows students (and the instructor) to learn more about each other.
Formulating effective questions is always a challenge. Questions which are too technical for the current course often cause students to flounder and provide very shallow answers. Questions which are not as technical, but more related to a real-world situation students are comfortable often lead to more thoughtful insights.
In terms of platform, I think I prefer a joint discussion board or blog for true discussion, however allowing individual blogs can be effective for courses in which students are writing a journal, creating a portfolio or logging research notes.