This study investigates the effectiveness of financial incentives (hereafter, simply incentives) in increasing viewership and social sharing of advertisements. Incentives can, in principle, help advertisers reach a larger audience, but there are two aspects of their application that have yet to receive much attention: (a) How large of an incentive will be needed to engage users with advertisements in the real world? (b) How does the design of a system that offers incentives in exchange for advertising exposure impact said incentives' effectiveness? To this end, we develop a mobile application that offers a user advertiser-subsidized access to data (and thus content) in exchange for exposing herself or her social network to advertisements. We conduct a randomized experiment in which users were offered a monetary incentive in exchange for engaging with an advertisement, watching or sharing the advertisement with their social network on Twitter. Our work speaks to two open questions in the networking and marketing literature: how can we design effective 'Sponsored data plans' and how can we convert individuals into agents of social sharing in 'Paid advertising.' Our initial results from the experiment show that users do respond to incentives, but they do so in a heterogeneous fashion, conditional on a variety of factors, including advertisement characteristics, incentive amount, and the type of engagement (i.e., watch vs. share).