The inspiration for Robert Putnam's famous book Bowling Alone was the observation that although Americans bowl at more or less the same rate as always, they increasingly are "bowling alone" rather than as part of organized leagues. Putnam sees this decline in participation in bowling leagues as suggestive of a general American social decline in the desire of individuals to join social groups, etc. "Bowling alone" is a case study of the phenomenon.
My cousin owns a bowling pro shop in upstate New York. Last year a law was enacted that prohibited smoking in bowling alleys, in all parts (including the lounge). In response, half of the bowling leagues at that alley folded (nearly taking his pro shop business with it, incidentally). Putnam is talking about the decline in community over a long period predating last year; nonetheless, I thought this story was an interesting example of the surprising and unintended social effects that can arise from a seeming unrelated regulation. More people are "bowling alone" in upstate New York this year than last, but it has little to do with Putnam's explanation.
Similarly, I recall that when I lived in Mississippi, one of my colleagues observed that he thought that one reason why "social capital" levels tended to be lower in Mississippi than elsewhere was the historic prohibition on the sale of liquor by the drink in bars and restaurants (lifted relatively recently). He hypothesized that this one law gave rise to a custom of entertaining in ones' homes, rather than in public houses like bars and restaurants. This, he believed, led to a general atrophying of the public sphere not only in terms of parks, but also in terms of lower levels of public trust and civic-mindedness. I don't know if it is true, but if so, it is another interesting example of the phenomenon. For what it is worth, when we lived in Mississippi we always went to friend's homes for dinner, which we do much more rarely in Northern Virginia. Such social cultures, of course, are highly network goods, and thus become highly path-dependent and difficult to later change.