in both dwelling types on their Facebook feeds, with an average frequency of 4.9 ads per person.<sup>5</sup> Two weeks after the end of this remarketing campaign, we sent them a follow-up survey (N=1,542) asking about bed net use and treatment seeking in case of symptoms. We find that the campaign was equally effective for users residing in both types of dwellings, reporting an 8-pp-increase in the probability of sleeping under a bed net the previous night, and a 2.5-pp-increase in timely treatment seeking in the event of a fever.

We conclude that the ad content itself was effective at informing people and persuading them to take preventative and response actions to fight malaria. The campaign was also effective at large scale for those it reached, but it did not sufficiently reach those most at risk, whose engagement on social media is lower and more expensive. This emphasizes the importance of accounting for algorithmic targeting in social marketing. Cost effectiveness analysis based on administrative data indicates that the costs per avoided (officially reported) malaria case range between $3.4 in urban areas and $6.5 overall. Given that the estimated number of cases is roughly twenty-two times greater than the amount of reported cases (WHO, 2021), it is plausible that the cost per averted estimated case could fall below $0.20. Altogether, our results suggest that social media campaigns with public health objectives are cost effective tools, yet they should employ more advanced targeting strategies to ensure ads reach not only those who are the cheapest to advertise to or the most likely to engage with the ad, but also those who are most in need of the campaign.

The rest of this paper is organized as follows: Section 2 describes the contribution to the literature and Section 3 discusses the intervention and its theoretical framework. Section 4 outlines the study design. Section 5 presents the study findings, while Section 6 disentangles the mechanisms of content effectiveness and targeting. Section 7 presents the cost-effectiveness analysis and Section 8 concludes.

# **2 Contribution to the Literature**

Over the last decade, a growing body of the marketing literature has studied the effectiveness of display advertising, finding positive impacts on both brand and performance marketing metrics (Bart et al. 2014; Goldfarb and Tucker 2011 _b_ ; Johnson et al. 2017; Lewis and Reiley 2014), and highlighting its inefficiencies and measurement challenges (Gordon et al. 2021; Johnson 2023). Our work builds and expands upon this literature providing new evidence on the impact of ads on societal outcomes beyond traditional marketing outcomes, and describing a new methodology to measure ad effectiveness on off-platform behaviors.

In this respect, the paper is close to recent studies assessing the effects of advertising campaigns with public health objectives, in both developed and developing countries. Digital and social media campaigns have been shown to be effective in reducing alcohol and tobacco consumption (Thompson et al. 2013; Wang et al. 2021), changing attitudes and behavior towards COVID-19 vaccination and routine immunization (Athey et al. 2023; Evans et al. 2023), promoting better physical and eating

> 5The ads show up on their Facebook timeline from the MNM page, like any ad, and respondents do not have any way of knowing that any particular ad came from the research team.

5