I think you are missing the point. I am 28 and an active member of the community.  I do personally know lesbians.  But whether its going out to dinner, or working at the Food Co-op, lesbians don't seem any more common than elsewhere in New York City.  There is ONE lesbian bar on 4th Ave.  

Park Slope IS NOT called Dyke Slope by anyone except perhaps as an insult.  You are missing the point. ITs a fiction!!!