Complex sentence: As of March 1,2009, over 23,000 people had volunteered with Common Ground Relief for various lengths of time, creating an unusual social situation in the predominantly black neighborhoods, since most of the volunteers were young white people from throughout the United States and Europe.
Split into simpler sentences: As of March 1,2009, over 23,000 people have volunteered at Common Ground Relief. Thousands of people volunteered for various lengths of time, creating an unusual social situation in the predominantly black neighborhoods, since most of the volunteers have been young white people from throughout the United States and Europe.