I know that there are other entries in the short collaborative fiction space, namely Novlet and Portrayl (both of which were profiled along with Ficlets by ReadWriteWeb a while back). But they hadn’t gained the same amount of traction and their user bases are still relatively small.
However, a new challenger has appeared on the scene called Protagonize, and according to Alexa, it has enjoyed some fairly interesting traffic numbers over the past couple of months. It has nearly the same premise, and a heavy userbase that appears to be constantly creating. It’s a chore to sift through the content at this point, there’s so much of it. But, I have to say, the quality of the writing so far is truly amazing to witness. I’ve been planning to get some more Ficlets written lately, and I might just have to start cross-posting and seeing what kind of a community builds around the content on Protagonize.