it was a "social experiment" since, unlike in most other places, a new nation was created in a short time from the many people who came to the land (unlike for example in Europe, where people have lived in the same place for generations).
So people could start the organisation of the society almost "from scratch" rather than negotiate slowly what was already threre - and this was new, an "experiment".I think that the experiment is in a way still ongoing, but not as much as it was before. The challenges that the US faces today are more similar to those of other nations.