We used the Mercy For Animals social media outreach budget to systematically estimate the efficacy of different advertisements on different demographic groups. Based on the click-through rates (CTR: the percentage of impressions resulting in clicks on ads), we observe:1. Animal-based advertisements are more than twice as effective as other types of advertisement, achieving a CTR over 3% compared to just 1.1-1.5% for environment, health, and social adverts.2. Females and older users were more likely to click adverts compared to males and younger users, respectively.3. Specific messages may outperform others within these broad themes. We find that pigs were the most effective animal advert (vs. cows, chickens, and fish), climate change was the most effective environment advert (vs. land use, water use, and deforestation), and chronic disease was the most effective health message (vs. obesity, pathogen contamination, and antibiotic contamination).The findings can be used to inform social media strategy for Mercy For Animals and the broader animal protection movement.