There are so many fun ways that you could use these sites. You could design reading posters and magazine covers. (using student pictures)You could help students create comic strips, post it notes, design logos for book covres or even create avatars. All of the things that they generate could be tied to a piece of writing they have done or the images could be used as a writing prompt. The possiblities and uses of these in the library are endless.
The only problem is that these site must be monitored! Several of them had chat rooms at the bottom that could contain improper information.
Here are some fun images that I created: