They still had third places where they could organize face-to-face. Think union halls, fraternal organizations (which us Millennials only know about from old cartoons), churches, etc.

See also this Adam Conover video, which isn't specifically about organizing to protest but nevertheless is pretty insightful about it.

they didn't use maligned, monitored areas for communication.... people today should coordinate face-to-face or at worst via telephone if they haven't already exposed themselves.