The Dow Jones Industrial Average, already down almost 20% in the past week alone, had another dismal session Thursday. It dropped 679 points, or 7.3%, to close at 8,579.19.

The Dow is at its lowest level in five years and 40% below where it was a year ago. Even in May, amid the disastrous housing market and the aftermath of Bear Stearns, the Dow managed to end several sessions above 13,000.

Yet the true reckoning will come with the upcoming quarterly earnings season. Usually the Wall Street ritual is far less interesting to showbiz than the Oscar or pilot seasons, but it has everyone’s full attention this time due to the economic debacle.

NBC Universal parent General Electric will report third-quarter numbers today, and the media congloms will follow over the next three weeks.

Damage on Thursday was broad and severe throughout the media sector. The valuations of many companies are alarmingly puny considering where they were just a few months ago. Some have reached multiyear bottoms.

The hit list includes CBS, off 11% to $10.14; News Corp., down 8.5% to $9.21; Time Warner off more than 6% to $10.09; and Yahoo down 8% to $12.65. Viacom, Disney and Sony were all down as well, though not as dramatically.

Time Warner has not seen comparable share prices since 2002, after the conglom took massive writedowns after the calamitous AOL merger. News Corp. hasn’t dipped below $10 in more than a decade.

The only modest gainers among major companies in the film and TV arenas were Regal Entertainment Group and Netflix. While both companies have had their blemishes of late (who hasn’t?), their gains add more fuel to the recession theory that movies, whether enjoyed in stadium seats or one’s couch, can be a countercyclical business.

Stakes are now especially high given the meager share prices, which, along with frozen credit markets, will impair dealmaking — or simple value creation — for some time to come.

The stocks of many big companies outside the studio gates are being watched anxiously as well.

General Motors is trading at 1950 levels, and Ford is at $2.08 a share in the wake of an eye-popping 30% year-over-year drop in vehicle sales in September.

The beleaguered state of major ad buyers such as the automakers is one of many reasons for mounting angst about the advertising biz. Upfront TV ad commitments rose last spring, but the consensus is that many planned buys will be scrubbed, especially if earnings are weak.

Network execs are bracing for the prospect of a significant downturn in ad spending next year given all of the economic turmoil. So far ad commitments for the fourth quarter are on target, and there hasn’t been a wave of last-minute cancellations, insiders at the Big Four nets say.

But there’s little doubt that automotive advertising will take a hit next year given the sales slump and the credit crisis. Net sales execs say the third-quarter earnings reports will be an important bellwether of spending plans for the next few months, particularly the results for major retailers and packaged goods concerns.

Already, some advertising prognosticators are revising down their spending projections for 2008 and 2009.

Global ad sales giant ZenithOptimedia got the biz’s attention earlier this week with a revised forecast that cut its projection for the growth of all ad spending in the U.S. by half, or down to 1.8% from the 3.5% growth it predicted in June. Zenith also projects an anemic 0.9% growth rate for the U.S. next year.

The situation for local TV stations is pretty dire at the moment, with key local blurb-buyers like car dealerships and banks feeling the sting of the market turmoil.

On the national front, the mainstream broadcast and cable nets are relatively well positioned to weather a recession, industry execs maintain. If marketers pare down budgets, digital and new-media platform spending will be the first to go, in many instances.

“When times get tough, you fund the tried and true, and there’s no better way to reach a mass audience in a timely manner than national TV,” said one Big Four network sales chief.

(Cynthia Littleton contributed to this report.)