In my larger classes, students often are reluctant to talk in front of the whole group. That same type of shyness can lead to a very quiet discussion board. Students might also neglect the online forum because, in their mind, it is something superfluous, not really the class per se, but rather some kind of separate, peripheral thing that can be ignored if they so chose. If the instructor tends to feel the same way, consciously or subconsciously, students will detect this attitude quickly.