Booth has a pretty huge alumni base in NYC. The school is always hosting events for alums and current students in Manhattan. I know that NY alums are very involved in on campus recruiting, especially for banking and consulting (two industries that bring a lot of Boothies to NYC).
I don't know as much about the alum network in Southern Cali. I know it's there and that Booth also hosts events in the city but I don't know if it's as robust and active as NYC's. Is there something specific you're looking to use the alumni network for?