As a U.S. native... well, I think it is both. I think "cultural imperialism" is a strong and politically charged term though. I don't think Hollywood was trying to dominate the world, I think it was just trying to make money.

Also, although I think Hollywood is still an important center, it is not really the same as it was in the days when the phrase "The Dream Factory" was coined. 

Do you know the story of why film production became centered in Hollywood? It started around 1912 or so and it has to do with patents for motion picture cameras, specifically the "Latham Loop," the little loop of film in between the sprocket and the gate. At that time, film production was centered around New York City, and outdoor scenes, such as Westerns, were shot in the New Jersey Palisades. 

A group of camera manufacturers owned the patent rights and charged huge amounts of money for their cameras. Many movie producers were unwilling to pay the price and were using unlicensed cameras. If they had been caught, they would have been taken to court and sued. To avoid this, they moved west. Of course, the reliable outdoor light also helped.