YouTube, that incandescent tower of video Babel; monument to the sloughed-off detritus of our exponentially-exploding digital culture; a Technicolor cataract of skateboarding dogs, lip-synching college students, political punditry, and porn; has reached the zenith of its meteoric rise; and Icarus-like, wings melting; is spiraling back to earth. Despite massive growth, ubiquitous global brand awareness, presidential endorsement, and the world’s greatest repository of illegally-pirated video content, Google’s massive video folly is on life-support, and the prognosis is grave.

Believers would have us think that Google (GOOG) will sustain YouTube, indefinitely if necessary. Proponents of online advertising argue that increased understanding of the medium will lead to more advertising dollars at better CPMs, lifting all boats in a sea of monetization. In the short term, however, neither celebrity presidents, a rabidly growing customer base, nor a brand which has in three short years injected itself into the global cultural lexicon can forestall the inevitable: YouTube is soaring towards the future like a pigeon towards a plate glass window.

The problem lies with the bean-counters. According to a report by Credit Suisse, YouTube is on track to lose roughly $470 million in 2009. No matter Google’s $116 billion market cap: a half-billion dollar loss on a single property, even one as large as YouTube, is a bitter pill to swallow. Even Eric Schmidt, talking to the New York Times about the YouTube acquisition, was quick to say that, going forward, Google would “be more careful with potential large expense streams, which are of uncertain return.”

Credit Suisse estimates YouTube will manage to rake in about $240 million in ad revenue in 2009, against operating costs of roughly $711 million, leading to a shortfall of just over $470 million. This half-billion dollar loss comes after more than a year of feverish experimentation in various forms of advertising, cross-product embedding, licensing and partnership deals. YouTube is adamant that ultimately they’ll find an advertising solution that will enable the ungainly behemoth to reach profitability. Looking at the math, it doesn’t seem likely.

The economics are hard to overcome. Assuming YouTube delivers the 75 billion streams that Credit Suisse projects for 2009, and assuming YouTube manages to slot an ad for every stream (which is practically speaking, impossible, given the nature of much of their content), YouTube would have to achieve a $9.48 CPM for every video impression shown. Presumably, the videos YouTube is already monetizing represent the best content available, with diminishing returns as they reach deeper and deeper into a repository rife with copyright violation, the indecent, the uninteresting, and the unwatchable. Hulu claims to be charging a $30 CPM, of which roughly 70% goes to the copyright holder. Averages for other proprietary content hover around the $10 CPM mark. CPMs for user-generated content, assuming you can attract the advertisers, tend to be measured in fractions of a dollar.

So what does this mean? It seems safe to assume that YouTube’s traffic will continue to grow, with no clear ceiling in sight. Since the majority of Google’s costs for the service are pure variable costs of bandwidth and storage, and since they’ve already reached the point at which no greater economies of scale remain, the costs of the business will continue to grow on a linear basis. Unfortunately, far more user-generated content than professional content makes its way onto the site, which means that while costs grow linearly, non-monetizable content is growing geometrically as compared against the monetizable content that YouTube really wants and needs to survive. This means less and less of YouTube’s library will be revenue-contributing, while the costs of delivering that library will continue to grow.

With the ongoing hammering of ad CPMs and unstoppable growth in the site’s popularity, Google is going to bleed substantial cash on this experiment for the foreseeable future. With costs of operation at half a billion dollars and growing, YouTube’s future is very much in doubt.

What are Google’s options? They seem unlikely to sustain a billion-dollar annual experiment with no path to revenue, no matter how much they paid for the original asset. In an organization feeling the sting of layoffs, is this really where Google wants to spend its money? It all depends.

Google could take a lesson from its neighbor, Hulu, and focus only on proprietary content with existing consumer loyalty and real monetization prospects. With its massive audience, this is a viable option, and a direction in which YouTube has already taken some baby steps. Axing user-generated content would seem to be anathema given the site’s roots, but it may be the surest way of putting the business into the black.

Alternatively, YouTube could implement a subscription structure for the site, either monetizing certain members-only content, or requiring users to create a paid account in order to contribute content. With so many marketers looking at YouTube as part of their viral strategy, this too could be a viable option.

One thing is clear: YouTube cannot maintain its current course and remain a going concern. Google can continue to fund the experiment for a period of time, but at some juncture, shareholders will ask hard questions about why Google is sacrificing half a billion dollars to support a project whose chances of providing a return, at any point, is dubious at best. Advertising cannot solve the problem, at least not in its current form, and not in the near term. With a diminishing field of options, a massive, growing, cost center, and an economy in recession, Google will need to make some hard decisions about the future viability and business model of its prodigal child.

Benjamin Wayne is CEO of Fliqz