Just wanted to echo this. Moved from Russia to Atlanta, and did not go out of my way to do the whole immigrant community thing (unlike what a lot of my parents and their friends did). 9 years later, living on West Coast now, and I definitely attribute my proficiency in English to not joining the Russian expat community. Meanwhile, there are people I personally know who were BORN in the US in one of those communities, and they barely speak more than just the very basics, despite their community not being an isolated one at all.

P.S. still have an accent, but it has been getting much better, and almost no one manages to guess where it comes from correctly. Heavy majority of the guesses go in the direction of Scandinavian countries :)