Public school -- new concept that caused massive changes to social behaviors and patterns of association. Nobody knew ahead of time what the results of this would be, so it was as much an experiment as letting kids access Instagram.

Rock & roll -- millions of people thought that letting children listen to this African American music would corrupt their children and prove ruinous to children. Some parents demanded controls on access to this music, much as some parents are doing with Instagram today.

Desegregation -- it's well worth an hour of reading if you haven't spent the time so far. Here I will just say that it was obviously a profound change in social patterns, changes to the US's legal caste system, and had to be enforced at gunpoint. Nobody knew how it would play out, like nobody really knows how using Instagram at age 15 will impact people in midlife. This led to (white) parental outcry over the prospective changes, as many (white) parents did not want to participate in this social experiment.

Big changes, no real control groups, unpredictable outcomes. Experiments in a very real sense.