The widespread diffusion of powerful mobile devices with diverse networking and multimedia capabilities, and the associated blossoming of content-centric multimedia services is contributing to the exponential increase of data traffic in cellular networks. Mobile data offloading is a promising technique to cope with these problems, which allows to deliver data originally targeted for cellular networks to complementary networking technologies. Among the various forms of mobile data offloading in this study we focus on offloading through opportunistic networks. Differently from previous studies in this field we evaluate the efficiency of opportunistic offloading schemes by using a real cellular traffic dataset collected in a large metropolitan area over a period of one month. We focus our analysis on video requests for popular video providers, and we evaluate the potential benefits of using an opportunistic data dissemination scheme to request this videos from local users instead of using the cellular network. As a benchmark, we compare the performance of such system with a simple caching mechanism. We show that a simple opportunistic offloading scheme can improve the performance of the caching system even if only 10% of the users participate in the opportunistic dissemination. This means that operators could offload their network efficiently without needing to deploy additional caching infrastructure.