In reply to Jim Fraser:
I haven't any figure to give but an awful lot of people went there to flee oppression... many from the old Austro-Hungarian empire, fleeing poverty in S Italy, Armenians and Greeks from massacre in Turkey (have you seen the film "America, America" by Elia Kazan?) all in the 19th and 20th century. The original "Pilgrim Fathers" were protestants fleeing religious persecution, or Jews or Scots chased from their land in the clearances, then Irish by famine, to give just a few examples.
The USA wanted people to fill it's "empty spaces" (the indigenous population wasn't considered, a bit like nowadays in Palestine before someone points it out) and also for the factories of the industrial revolution. Life was hard for working people in the "old world", even agricultural workers in Britain were paid barely enough to survive even when in work, and this was one of the more prosperous countries.
I don't think that many went there, a long and dangerous voyage into the unknown, often ending in death by disease (Dickens paints a picture of this in one of his books, can't remember which, where the "hopefuls" were pitilessly taken advantage of by those already established there) without a pretty strong motive.