This study proposes affordances for discursive opportunities (ADO) as a theoretical framework that leverages the concept of technological affordances and the theory of discursive opportunities to understand platform potential in shaping social media activism. Specifically, ADO underscores how social media platform affordances (e.g., algorithmic curation, shared group identity and culture, connectivity) shape movement visibility, resonance, and legitimacy, all of which can mobilize activism efforts. We further situate our discussion in the cross-platform context to explore both differences and interconnections between different platforms in facilitating digital activism. By analyzing 20,363,128 English posts related to the #MeToo movement from 2017 to 2020 on Twitter, Facebook, and Reddit using supervised machine learning and time series analysis, we reveal platform variations in affording discursive opportunities for visibility, resonance, and legitimacy, which shape activism efforts differently across platforms. We also find a unidirectional relationship from activism on Twitter and Reddit to activism on Facebook.