You see a lot more activity if you start the forums and post the first discussion topic. Adding your input to discussions is also valuable because it shows learners that you're engaged and are reading their postings. If learners find replies interesting and useful, they're more likely to get involved in the forum discussion.
You can make forums a required task as part of the course and assign grades, points, or scales. Moodle pushes the marks straight to the grade book, which ensures participation.
Set up a forum leader/monitor, or let learners take turns and share the responsibility. You can set up a rubric (a set of rules for assigning grades for scoring) and give learners capabilities to rate or grade postings.
The following list details other ways that you can creatively use forums to keep your learners engaged: