Do social media help individuals without organisational memberships to engage more in politics or do they only facilitate political participation for those already involved? We examine how social media use and organisational membership jointly affect participation. Comparative surveys in Hong Kong and Taipei reveal that information sharing and virtual political engagement on social media mobilised users to engage in collective political actions. The influence of social media on individual-based participation is conditional on organisational membership, as reflected by the number of organisations joined. Organisational membership moderates the relationship between social media use and political behaviours differently in Hong Kong and Taipei.