Social media interventions are usually evaluated in individual-level experiments, which are costly to scale and often miss collective effects. We propose the Exposure--Engagement model, whose parameters separately control what social media users are exposed to (recommendation) and how they engage (behaviour). After fitting it to the Italian Twitter debate (2018--2022), we validate it at individual and collective levels. We simulate behavioural interventions (prebunking and accuracy prompts), calibrated on effect sizes, and a recommender-system intervention (chronological feed), comparing effects on engagement, attention diversity, and segregation. All three reduce engagement. Behavioural interventions slightly reduce diversity, leaving segregation unchanged, whereas the chronological feed increases diversity and reduces segregation. For these interventions, changing recommendation does more for the debate's structure than changing behaviour. We design a recommender-system intervention that increases engagement and diversity while reducing segregation, although with about a tenth of the chronological feed's gains, showing the framework's potential for pro-social intervention design.

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