The United States is an assimilationist society because people from different minority groups and nations of origin tend to meld themselves into the dominant culture and become Americans.
In a pluralist society, people of different groups do not blend into the mainstream over time. They remain as separate groups that do not participate in the dominant culture. If they do participate in the dominant culture, they do so as something of outsiders, retaining something of a dual identity in which they see themselves as fundamentally different from the dominant group.
In an assimilationist society, the outsiders become insiders. They no longer feel that they are different from the dominant group. In American terms, they stop feeling that they are (for example) Filipino-Americans and start feeling that they are simply American.
In the US, we have aspects of assimilation and pluralism. However, many people do fully assimilate into our culture. They do so because our culture is generally a welcoming culture that will accept anyone as a true American so long as they wish to become that. By the second generation, most immigrant groups have completely assimilated because our culture allows that to happen and because our culture is an attractive one that the vast majority of immigrants would like to be a part of.