Instant Multimedia Messaging (IMM) applications are increasing their popularity in cellular networks, rapidly taking over the traditional SMS and MMS messaging service. This paper presents the first large-scale characterization of WhatsApp, the new giant in IMM. Understanding how it works is paramount for cellular operators and service providers, both to assess its impact on the network as well as gaining know how for tracking its growing usage. Through the combined analysis of passive measurements at the core of a European national-wide cellular network, geo-distributed active measurements using RIPE Atlas, live traffic captures at end devices, and subjective Quality of Experience (QoE) lab tests, our study shows that: (i) the WhatsApp hosting architecture is highly centralized and exclusively located in the US; (ii) multimedia sharing covers about 75% of the total WhatsApp traffic volume, with 36% of it being video content; (iii) flow characteristics depend on the OS of the end device; (iv) despite achieving download throughputs as high as 1.5 Mbps, about 35% of the total file downloads are potentially badly perceived by the users, showing the impacts of the long latencies to WhatsApp servers. Our analysis additionally overviews the worldwide WhatsApp outage occurred in February 2014.