They have a business model. It just takes capital to spin up new markets. (They run a market at a loss initially, until its mature enough to take profit from.)
What we know from observable evidence is that they're raising tons of money, and funneling it into growth. We also know that there are lots of subsidies, on both sides of the market -- even in San Francisco, the most mature of Uber's markets, I regularly see and hear of driver referral bonuses.
It could well be that without the easy money, the Uber story becomes a lot less compelling. Time will tell.
http://www.businessinsider.com/uber-revenue-rides-drivers-an...
One of the ironic things about Uber is that after they overcome regulatory hurdles in new markets, they make business dramatically easier for their competitors. Taxi service without regulatory capture is a commodity product.
http://www.fastcompany.com/3055033/instacart-raises-fees-and...
It isn't 1999 again, but I wouldn't go so far to assume that "most" of the giant companies are solid businesses just yet. As Buffett said, you don't know who is wearing swim trunks until the tide goes out. Even if companies aren't obviously subsidizing their customers' purchases (as in the first boom), there's a lot of bubble money sloshing around, making everyone's revenue numbers look better.