One of the ways that I have tried to get students to write a bit more is to use Wallwisher in class. Though we haven't used it for awhile, I found that most students were receptive to the idea of writing short messages about their thought on a topic such as world hunger.
When I do this again, I'll have to set up separate Wallwishers for each class since I've got 35 in each of my geography groups. As you can see, the wall just gets too crowded.
Tom Barrett also started a Google Presentation on "Interesting Ways to Use Wallwisher." Check it out for a gold mine of ideas.