Workforce diversification is essential to increase productivity in any world economy. In the context of the Fourth Industrial Revolution, that need is even more urgent since technological sectors are men-dominated. Despite the significant progress made towards gender inequality in the last decades, we are far from the ideal scenario. Changes towards equality are too slow and uneven across different world regions. Monitoring gender parity is essential to understand priorities and specificities in each world region. However, it is challenging because of the scarcity and the cost to obtain data, especially in less developed countries. In this paper we study how the Facebook Advertising Platform (Facebook Ads) can be used to assess gender imbalance in education, focusing on STEM (Science, Technology, Engineering, and Mathematics) areas, which are the main focus of the Fourth Revolution. As a case study, we apply our methodology to characterize Brazil in terms of gender balance in STEM as well as to correlate the results using Facebook Ads data with official Brazilian government numbers. Our results suggest that even considering a biased population where the majority is female, the proportion of men interested in some majors is higher than the proportion of women. Within STEM areas, we can identify two different patterns. Life Science and Math/Physical Sciences have female dominance, Environmental Science, Technology, and Engineering majors are still concentrated towards men. We also assess the impact of educational level and age on the interest in majors the gender gap in STEM increases with the women's educational level and age, as confirmed by official data in Brazil.