As the economy has ground to a halt, venture-backed initial public offerings, one of the key indicators of Silicon Valley’s health, have done the same. According to the San Francisco Business Times, only six venture-backed IPOs debuted in all of 2008, marking the lowest point since the 1970s, when the industry was in its infancy. As we’ve argued elsewhere, many Internet start-ups haven’t been too panicked by the downturn, since cloud computing has lowered their operating costs and stretched out their burn rate. But there’s a limit to how long even sites like Facebook can keep burning cash. Silicon Valley has been the country’s most impregnable industry, except possibly for debt collection. If this keeps up, even the tech geeks will learn what reality feels like.