Its a difficult thing to analyze because there are multiple factors for our social circle and bias is only one of those factors. I'm a russian immigrant. Not only did I not have black friends, for the longest time I didn't have american friends and I went to school in philadelphia. I hung out with other immigrants from a similar part of the world because we shared a common language, culture, and general preferences. When immigrants come to this country, they usually become parts of immigrant communities. If I met a black person from russia, chances are I would have been friends with him sooner than a white American.