Via Matt Yglesias on Twitter:
This is really not surprising. Church buildings are almost optional by definition to begin with: as long as it was built properly in the first place and hasn't burned to the ground, you can generally make do with what you have rather than add on or tear down and start over. I also would be willing to bet that a good deal of the building in recent years has been done by large congregations, which tend to build, well, large buildings. It wouldn't take much of a slowdown to affect the slope, if that's the case. And then too, many large congregations have been getting into revenue-generating enterprises--everything from catering to real estate--so if those businesses have been hurt by the recession, it'll show up in this graph.
But the bottom line is that churches are no longer as recession-proof as they used to be. Don't I know it.