In the episode, america is referred to as a "social experiment." what do you think this phrase means? do you think the "experiment" is still ongoing?

This is a bit of a subjective question, but most would agree that America is indeed an "experience", since it is a "melting pot" of a plethora of different cultures--all living in close proximity in a way that it unmatched in practically any other country. And because there still exist great cultural and racial tensions, it's fair to say that the experiment is ongoing.