I think the level of activism in our bicycle community has to do with the fact that a lot of people that are activists for other causes, (global warming, peak oil, land use, etc,) ride bicycles because it is a good solution to those problems. As such, a lot of bicycle activism isn't just to promote bicycling, (although, I admit, bicycling is fun too,) but it is part of a solution to bigger problems... I make the comparison to some of my friends that care about those problems as well: those problems are "easier" to not think too hard about, and just drive a car instead, and those people are more in the "I should do something" than the "I am doing something" category... As such, I'd say that simply riding a bike is a form of activism in itself, and it is a very small step to go from there, to showing up at a few meetings...

There's also a study or two that shows a direct correlation between the amount of social interaction one has with their neighbors and the level of volunteerism.

While not necessarily my "neighbor" per se, I often will chat with another biker when at a stop light, or nod, smile, or say hello as I pass a biker or one comes by the other way. Observation leads me to conclude that many other bikers do this as well. Biking, by nature, allows and even forces you to be socially involved with other community members, whereas a driving commuter is encased in an isolated chamber, completely cut off from the their local community while they are driving.

While only speculation, one could deduce that not only is the rate of volunteerism decreasing in proportion to increased commute times, but also due to the increased social isolation caused by said commute.