In DC, they started by placing bike share stations near Metro (subway) stations. As the program's popularity grew, they expanded into some nearby suburbs, and are still expanding. These suburbs are generally pretty built up and Metro runs through them.

They're now expanding to areas that are not necessarily right on a Metro line. I think in general the new stations are close to bus stops if not Metro. They also have some physical requirements when choosing locations, such as flat ground and adequate exposure to sunlight because the stations are solar powered.

My new office is in a not-very-nice part of DC, with some gentrification going on nearby. The nearest bike share station is about 1.5 miles away, under a train trestle that delineates between the gentrified area and our area. However there is a college campus near here, on our side of the tracks, and I wouldn't be surprised to see them set up a bike share station near or on that campus.

I really don't know about the demographics of the bike share users here.