Jonah Lehrer (The Frontal Cortex) reviews a paper by Bahns, Pickett and Crandall on the relationship between social ecology (the make up of a community and its characteristics) and how people initiate and maintain relationships. They surmise that a bigger pool only makes us more picky, and keener to gravitate to people similar to us (the so called Similarity-Attraction effect).
The cliff notes version?
When opportunity abounds, people are free to pursue more narrow selection criteria, but when fewer choices are available, they must find satisfaction using broader criteria.
Perhaps LightherLamp put it more elegantly a few days ago. Full text of the paper here.