Stack Overflow has been nearly useless as anything other than an archive of old questions for a long time. It's a case study in how gamification can backfire.
For me it was over was before the peak as shown. The gamification that got it started in the first place eventually led to it becoming almost unusable for its intended purpose. Instead it functioned as more of an online-only augmented reality game where people worked to moderate away new questions as efficiently as possible rather than actually answer questions.