Some students may not perceive the value of participating in discussions, so you could choose to attach a certain percentage value to their online interaction by making it part of the course grade.
Another method may be through being present yourself, by posting messages and responding to students' questions and or replies. In this way you are modeling the type of behavior you expect from them. This can also create an atmosphere that will keep students coming back to read and post.
As an instructor, you can set some limits, encouraging students to restrict their messages to concise, well-thought postings of no more than a screen of text. Most often, large amounts of text frustrates students, producing an appearance of extra work and making reading difficult.
Lastly, be vigilant, if students are not participating, it may be a good idea to send them a private message through email to find out why.
Currently, the only way to change the dates is to re-post the message. Although, You can compile all messages and download them to your local computer where dates can be altered and uploaded again.
Private and anonymous settings in the Discussions area can be turned on and off at any time.