Forums are common features of online courses. The software is usually provided by the institution. It is in everyone's interest to make a course forum useful, maybe even lively; generally we want more posts rather than fewer, and more interaction between those posts, rather than a bunch of individual observations.
Using the forum is sometimes counted as part of students' grades. There are a great number of ways of measuring participation, though; I previously asked How to score class participation, correcting for natural outspokenness?
That said, making forum activity part of student grades is not necessarily key to making the forum useful. What is the most effective and practical way of getting students to post in the forum?