Today, more than ever, social networks and micro-blogging platforms are used as tools for political exchange. However, these platforms are biased in several aspects, from their algorithms to the population participating in them. With respect to the latter, we analyze the discussion on Twitter about an abortion bill in Chile, proposed in January 2015, and approved as law in September 2017. We find that Twitter has strong biases in population representation. Still, when carefully paired with demographic attributes, Twitter-based insights on the characteristics of political discussion match those from national-level surveys.