Yahoo-Tumblr ($1.1B, 2013)

Yahoo's $1.1 billion acquisition of Tumblr in 2013 suffered a 99.7% loss when it sold to Automattic for under $3 million. The deal imploded after Yahoo killed Tumblr's NSFW content, which drove a third of its engagement, slashing user traffic by 50%. This percentage loss is nearly double the 80% write-off of HP-Autonomy (#3), though Autonomy's absolute dollar loss of $8.8 billion remains larger. Marissa Mayer's strategic misstep remains a textbook example of destroying platform value through content moderation.