Free Technology 4 Teachers. This is a nice site for collaborative storytelling that reminds me a bit of Boom Writer. It is a social network that allows users to create a story from scratch and then either set it public to allow others to collaborate, or set to private to allow people you invite to collaborate. Once a piece is done it can then be voted on.
I recommend checking out Zopler by clicking here!!
For another nice site for collaborative writing check out Folding Story.