Today, Sept. 21, is World Alzheimer's Day. To mark the occasion, Alzheimer's Disease International is offering absolutely "nothing" in an effort to build awareness and empathy—via a Facebook app, developed by Ogilvy Belgium, that causes all of the information in your Timeline to temporarily vanish for the day. All of your posts and pictures are still saved, but inaccessible today and replaced by this message: "Imagine your life without memories. For 36 million people living with Alzheimer’s disease, this is reality." Now, this particular cause has long generated powerhouse public-service appeals, probably because the wrenching struggle lends itself especially well to dynamic storytelling and intense imagery. Yet, replicating the doubtlessly disorienting personal hell of Alzheimer's for those not afflicted remains, I think, a tough task. And this app might succeed in a quietly jarring way where louder, more dramatic efforts have fallen short. Facebook profiles are such an intrinsic part of some people's lives that "losing" years of deeply treasured cyber-mementos, if only for a day, could provide at least of small taste of what Alzheimer's patients endure. Or perhaps the approach is too subtle, lacking the visceral impact of quick-cut, effects-driven TV spots or the ah-ha illumination of clever print ads and Web reveals. Once the Facebook profiles return, will folks recall the experience and support the fight against Alzheimer's, or will the exercise almost immediately fade from memory? Download the app here. Via PSFK.