The discussion board is excellent tool to promote community building in online courses. Because learners have fewer chances to interact with each other, it's important for instructors to facilitate community building in their online courses whenever possible.
There are a number of activities that could promote community building in a discussion forum. The first, and most popular activity is an Introduction forum, where each learner writes a paragraph or two about him/herself. The instructor can leave it to the learners as to what information they wan to share, or they can ask for certain information, such as why they are taking the course, their reason for taking the course, what they hope to get out of the course, etc.
Learners can also share information with each other as a way of building a community. Instructors can create a 'resources' forum where learners can post links, documents, videos and other objects that may be helpful in completing assignments, or relevant to the course in general.
Another kind of community building forum I have seen is a 'cafe.' This was a forum that the teacher created for the learners to post anything not related to the course. Once the learners got to know each other, a kind of camaraderie began and learners posted job openings in the field, book reviews and even events.