On credit applications our business bank specifically asks if more than 25% of your revenue came from one source in the past calendar year.
If you think about it, a typical brick and mortar gets its money from hundreds if not thousands of customers every year (restaurant/cafe, auto mechanic, etc).
Your restaurant/cafe may have lots of customers, but they may be coming to you b/c you're across the street from the movies or a mall. If a primary store in the mall closes (or as in my hometown, a stabbing in the mall scares away customers), your business is pretty dead.
That's why the smart thing to do is open several locations, which is what restaurateurs generally aspire to, for that very reason. Sometimes it backfires, usually it doesn't if the restaurateur knows what they're doing.
Right, but these companies have lots of customers. They just get them from one place.
It's like renting a particular location: it's technically true that you could lose your lease, but that's only a risk to your business's long-term viability if your lease is artificially cheap (otherwise, you'll just move to another place on similar terms). And if your location is a particularly good deal, you should take it, even though losing it would suck.
Online, the same dynamic applies, but with lower transaction costs.
You can get revenue from many sources, as in many individual customers, but be dependent upon a single source for your product.
The situation of startups who depend upon an external large business like Facebook, Amazon or Twitter is analogous to a car dealership that only sells one kind of car - say, Saturns. If Saturn stops selling to you or goes out of business, you're going to have to make some major changes.
If you think about it, a typical brick and mortar gets its money from hundreds if not thousands of customers every year (restaurant/cafe, auto mechanic, etc).