Measuring the effectiveness of marketing efforts across various channels is a challenging task. Such measurement usually relies on a combination of key performance indicators (KPIs) used to assess marketing outcomes. In this work we present the Multi-channel Marketing Monitoring Platform (M3P). M3P harnesses the crowds (people) as sources for effective collection of marketing KPIs across all possible channels. We describe M3P's main challenges and characterize marketing KPIs. We then describe the M3P solution, focusing on its KPI extraction process. © 2014 IEEE.