WallWisher is not as useful pedagogically as other social networking tools I can think of.
You can add a picture (from a URL), or video, or audio file, but it doesn't have a way to put a URL into the text (without using up the tiny letter count), so that users can easily go visit the site. If you want to illustrate your sticky, you need another place/server/or knowledge of how to get an image location to stick it in. You can post to Facebook, et al., and link to pictures in Flickr.
The limited number of words/letters on a sticky means you can't say much--this is probably an advantage for young learners, but not necessarily for their education. A blog or wiki would give them the opportunity to be more expansive.
You could have a main sticky and then ask students to respond to it. But there is no way to organize the stickies besides moving them around physically. So this means the teacher/owner of the wall must do the work of organization, or let things happen at random. (See Nik Peachey's wall of teacher tools for social networking.)
So on the whole, I like the idea of a communal blog or wiki better: more opportunity to write expansively, ways to comment, RSS feed, easy ways to include URLs and lots of pictures, and esp. with a wiki, ways to organize pages.
But I can see Wallwisher as a way to make comments on an event or single Web page. You might have students visit a page or view a video (or put one into a head sticky), and then make a short comment on a Wallwisher page. If you have only 15-30 students, they could all read each other's comments (but if they then make more comments, you've quickly got a huge, disorganized mess...) If you use WallWisher the way it was intended, perhaps to wish a class member happy birthday, or to congratulate someone on getting citizenship, or to make a wish list, or to put up reminders for a project, then it's a nifty application.