It is not as if George Washington or the Civil Rights Movement recruited masses of followers online. They started with relatively small but cohesive units. Yes, it took them longer than it takes today to assemble a flash mob. But the efficiency measured by capability was higher than today.

For an example, if I were a German cop, I would be much more concerned about the mostly-offline groups such as the Reichsburger or the Grey Wolves (Turkish nationalists). They linger around for decades and are hard to penetrate, unlike whatever wild crowd you can put together on Facebook. That Facebook group will look scary, but it won't have much pull after 5 years, because it competes with a ton of other Facebook groups for attention.