People with invisible chronic illnesses (ICIs) can use social media to seek both informational and emotional support, but these individuals also face social and health-related challenges in posting about their often-stigmatized conditions online. To understand how they evaluate different platforms for disclosure, we interviewed 19 people with ICIs who post on general social media about their illnesses, such as Facebook, Instagram, and Twitter. We present a cross-platform analysis of how platforms varied in their suitability to achieve participants' goals, as well as the challenges posed by each platform. We also found that as participants' ICIs progressed, their goals, challenges, and social media use similarly evolved over time. Our findings highlight how people with ICIs select platforms from a broader ecology of social media and suggest a general need to understand shifts in social media use for populations with chronic but changing health concerns.