Immigration deals with immigrants who are human and with social values irrespective of their country, religion or sex. Of Course their individual background and societal value will count in integrating or non-integrating them into their immigrant countries (USA) and positively or negatively affect the formation of social and human capital in USA.  

I think one of the key issues for immigrants is the process of acculturation in their host countries that happens at a social and workplace levels. There is extensive research on the dynamics of this process, I did a brief summary and would be more than happy to share. 

The fact is new immigrants come into a country with established values, skills and culturally unique ways of doing things. As people, they boost the skill level available in the host country while also enriching its social fabrics by their uniqueness. This are some of the factors that tends to make the US seems more successful in immigrant establishment and acclimatization. I have written one or two articles on immigration, but  with more emphasis on the need of my country Nigeria and Africa in general..

The most extensive study on the effects of immigration and diversity on social capital is that by Robert Putnam, and his conclusion was that diversity decreases social capital: http://www.boston.com/news/globe/ideas/articles/2007/08/05/the_downside_of_diversity/?page=full