Online activity is often cyberbalkanized, but it remains unclear whether this phenomenon leads to polarization of public opinion or if the relationship works in the reverse direction. This study tested the temporal association between cyberbalkanization and opinion polarization during the debate on political reform in Hong Kong. Online communities were constructed by a post‐sharing network of 1,644 Facebook pages (101,410 shares); the differences between intra‐ and inter‐community shares were derived, and a cyberbalkanization index was computed. A time‐series analysis showed that the index temporally preceded the opinion polarization, i.e., most of the opinion poll's respondents gave extreme ratings to government leaders, but not vice versa. The index was particularly predictive of polarization among youth.