This is a misunderstanding of where shareholder value comes from.
The valuation (market cap) of a company is a reflection of the value of the assets that the company owns. That includes IP, trademarks, brand assets (somewhat perversely, this also includes user habits), business relationships & contracts, any resellable physical goods or cash that the company owns, and all future cash flows that will accrue to the company. Basically, the stuff that an employee, supplier, or customer could not take with them if they chose to do business with someone else.
Now, there's also the concept of economic value, which is the value that customers of a company (or industry) are willing to spend on its product. This can be reasonably proxied by the total revenue of the industry - for your GM + Ford + Honda example, that's about $150B each, or $450B (and leaves out the 800 lb. gorilla, Toyota). That's a lot larger than the total $60-80B market for all brand advertising.
But building cars has a long & large value chain. GM + Ford + Honda can't do it all on their own. They each employ roughly 200,000 people, many of whom are relatively highly paid thanks to union agreements. They have literally thousands of suppliers that they buy parts from. These suppliers themselves need to buy parts and supplies from labor-intensive industries like mining. As a result, the company captures comparatively little of the profit from all of this work. Facebook, for example, makes about $6B in net income off $20B in revenue, while GM makes $10B in profit off $150B in revenue. This also implies that further growth will benefit companies like Facebook or SnapChat much more than it would companies like GM: if margins stay the same, Facebook gets to $10B in profits with $30B in revenue, while GM would have to get to $300B in revenue (probably not happening) to get to $20B in profits.
It's a plus, from a shareholder value perspective, that SnapChat only employs 200 or so people, and their largest expense is paying Google for AppEngine bills. It means that whatever value SnapChat creates, the shareholders will capture almost all of it. Indeed, based on how often car commercials appear on TV, it's likely that a good portion of GM & Ford's revenue will become SnapChat's profit, just like how a good portion of SnapChat's revenue becomes Google's profit.