It seems hard to believe, but Google (NASDAQ:GOOG) (NASDAQ:GOOGL) acquired its hugely successful YouTube unit nearly eight years ago for what was then an unheard of $1.65 billion. At the time, YouTube was easily Google's largest acquisition, and the plan was to, "pursue our goal of building the next-generation platform for serving media worldwide." Safe to say, Google nailed that one.

At least we think Google hit a home run with YouTube based on estimates and analysis by various industry pundits. There's no way of knowing specifically how successful YouTube is because Google doesn't break out its particular financials.

But as Google's primary digital ad competitor Facebook (NASDAQ:FB) is beginning to discover, video ads offer tremendous growth potential, meaning YouTube's aggressive forward revenue estimates, if their anywhere close to accurate, are going to become more and more significant. With so much growth already, and much more to come, isn't it about time Google shared YouTube's financials with the world?

Word on the Street
Toward the end of last year, the general consensus was that YouTube would generate upwards of $5.6 billion in revenue. As if that weren't good enough, it was widely assumed that YouTube was continuing to grow, even more rapidly than Google itself, so who knew what 2014 would bring? Turns out, estimates from last year were, based on new analysis, partially correct.

Though it doesn't appear YouTube hit 2013 revenue estimates of $5.6 billion, a Jefferies Group analyst recently shared his assessment of YouTube's revenue and annual growth rate, and said he expects about $5.9 billion in revenue this year. If so, that would be a 48% jump from his 2013 estimate. As for next year, YouTube should generate a shade under $9 billion in annual revenues. As a point of reference, Facebook generated just over $10 billion in its last four quarters.

With a trailing annual revenue total of over $63 billion, this year's $5.9 billion in YouTube revenue is beginning to become a significant piece of Google's revenue pie. As for the future, it won't be long before YouTube makes up 10%, and soon even more, of Google's total revenues. In fact, Youtube should breach that benchmark by 2016 or 2017, according to the Jefferies analyst.

According to estimates, YouTube is growing considerably faster than Google itself, which is why it will make up a larger and larger portion of Google's total revenues. YouTube's annual growth rate this year is more than twice Google's Q2 overall revenue gains of 22%. Not only is YouTube already a significant part of Google, it will become even more critical going forward.

What's the big deal?
Some Google watchers may not care how much YouTube is contributing to its bottom line. As long as Google keeps growing revenues, who cares? Investors should care. The value of a prospective investment's individual properties, especially as they grow in value and importance, is a critical aspect in making informed buy or sell decisions.

Not knowing where a company's revenues are coming from is a worrisome proposition. How do you accurately evaluate the potential for future growth without knowing where revenues are derived currently, and in what proportions relative to various business units? The answer is: you can't, at least not as comprehensively as a prospective investor should.

Another reason Google should seriously consider breaking out YouTube revenues for the world to see, and evaluate, is that as a public company, sharing the quarterly and annual results of key business units is simply the right thing to do.

Final Foolish thoughts
Google is not alone in not disclosing each business unit's revenues. Will Facebook break out its Instagram, Oculus, or WhatsApp revenues by unit when it finally monetizes its newest properties? Only time will tell, but if they become as much a part of Facebook's business as YouTube is to Google, it should. As a publicly traded entity, Google has to disclose certain aspects of its business as per SEC guidelines. Obviously, sharing YouTube-specific revenues isn't one of the items the SEC is concerned with, but Google's investors should be. So come on Google, what's YouTube really worth?

Tim Brugger has no position in any stocks mentioned. The Motley Fool recommends Facebook, Google (A shares), and Google (C shares). The Motley Fool owns shares of Facebook, Google (A shares), and Google (C shares). Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.