As unpopular as this might be to say, and as environmentally unfriendly as it is, I've known my neighbors in every house I lived in; even when my friends and I rented a house in college, and after I was done with college, and I've rented houses with friends. They weren't always thrilled to see renters move in, but when we mowed the grass or were grilling, etc. you're forced to look over the fence and say "hey". Apartments offer a sense of odd anonymity, where very few know each other unless they already do, for some reason, especially in more urban areas. I was never more anonymous than when I lived in a city proper, living in an apartment building. The only people who knew my name were my coworkers.

EDIT... atrocious grammar