Can you describe a required online science course that used discussion well?
The best experiences I have had in working in group, in terms of exogenous motivation, were encouraging competition: define a problem, divide your students into groups, set a time limit and give a reward to the first/best/any-criteria-you-consider group. 

Live discussions/meetings can be more helpful that a forum in this sense: the probabilities of engaging in a fruitful dialog increase. 

You may also want to take a look to gamification techniques: 


http://en.wikipedia.org/wiki/Gamification
http://en.wikipedia.org/wiki/Gamification_of_learning


If students are not motivated by themselves, the internal pulse of "trying to win" at something may do it, gaining knowledge along the way, without having the feeling of "doing homework".