Once upon a time a unicorn fish was laying eggs. Some of the eggs were laid in the ocean, some others in a small pond up some river.
Half of the eggs in the ocean were eaten immediately, and the other half starved to death because their food was eaten by other fish. The eggs in the pond however hatched into little unicorn fish that were bigger than anything in the pond. They grew bigger, had eggs of their own, and dominated the pond. Eventually they expanded into the ocean, and changed the way other fish evolved.
Historians have quoted the residents of the ocean as saying "I can no longer live where there are no unicorn fish around; my existence and the future of our fish civilization depend on them." When asked why they killed the original eggs in the ocean, the residents exclaimed "Oh no not us! We liked unicorn fish before they were cool." 
Facebook didn't start out as the global social network; it started out as the social network for Harvard students (Small pond). Quora didn't start out as the question and answer hub for every topic; it started out from a very niche set of topics mostly to do with entrepreneurship, and the best experts in those. Google didn't start out as the search engine for everything on the Internet; it started as a tool to rank PhD research papers by citation popularity. Apple didn't start out as the best designer of computers for the world; it originally just wanted to impress a room full of geeky circuit-board enthusiasts.
In most cases, the founders of wildly successful startups didn't actually intend to use the small pond as a beachhead strategy to dominate the ocean. Instead, they were driven by the geeky lure of some shiny new technology, and honestly couldn't think of a larger market for it than the small pond they were already in:
Zuckerberg's ambition really was just Harvard at first. Larry Page's ambition really was PhD papers at first. I'm not sure about Adam D'Angelo, he could be a mad scientist; but even Steve Jobs simply wanted to sell a few Woz-made computer units to the computer retailer dude (Paul Terrell, founder of the Byte Shop) that he knew along El Camino Real road running down the San Francisco Bay Area. He had no clue back in that December 1975 that he was building the world's largest corporation.
The very new technology that drives the tinkering in the small pond also increases the size of the pond to an ocean. Facebook didn't get people to start connecting online. Google didn't get them to start searching online. Quora didn't get them to start asking and answering questions online. Apple didn't invent accounting chores or the desire to download MP3s. In a way, people were already using these services. The services just sucked, because they hadn't been touched by the new paradigm shift of technology. Incumbent players could see these shifts. But because they lived in the ocean, they couldn't plant the seeds of change:
Yahoo had search; which didn't work well because they were already better with manually categorizing all of the known Internet. Google could only have started in the small pond of PhD papers.
Xerox invented everything in the consumer computer, including the mouse and display screen; which didn't get put together well because Xerox was already successful in the business ocean. Apple could only have started in the small pond of a room full of geeky and enthusiastic consumers.
Today if you're offering a new service that looks like the future but gets no love, you should look for the small pond.
The "small pond" is not a demographic, a well-defined market vertical or a group of people forced together. When there's a driving technology making a new future possible, the incumbent often ignores it. Ignoring the future pisses off visionary customers and makes them disgruntled to the degree that they form their own enthusiast groups. These groups only have one thing in common: vision. You must find them.
Almost the only way to market to a group of visionaries is to put forth a bold vision and let them find you. A visionary doesn't want to get sold to; he wants to tell you what should be sold. So instead of looking for channels through which you introduce your product to them, you must look for avenues through which your ideology is presented.
If you go back to some early Apple ads and marketing campaigns like "Think Different", they're as much about ideology as product: "Here's to the crazy ones. The misfits. The rebels. The troublemakers. The round pegs in the square holes". Apple wasn't appealing to the quality of their product or the mass market. It was appealing to the small pond that was extremely dissatisfied with IBM's lack of vision about consumer computing.
Apple 1984 ideological ad against incumbent IBM
It turns out that we are all crazy. By the time IBM acted to catch up with the exponentially growing size of Apple's small pond, they did so with such haste and aloof negligence that they handed their incumbent monopoly on the PC market (still a relatively small pond) in a badly written contract to a then-unknown company called Microsoft. In that single act, they proved that with all its resources, a large incumbent always underestimates the small pond.
Apple finally caught up with Microsoft, the illegitimate son of the blind king IBM, when Steve Jobs bet the company on the small pond of pocket-sized computers like the iPod and iPhone. Google did the same when they made the Browser instead of Windows their Operating System. (Timing matters. Apple had many false starts.) It's hard to believe today, as I'm writing this on an iPad, that the then-insane decision to stop chasing the PC ocean and go after the elusive mobile small pond must have been made by a bunch of people around a white board. That decision is now worth almost 10% of most Americans' 401K retirement stock portfolio, whether or not they realize it.
The small fish gets eaten in the ocean. To invade the ocean, start from a very small pond, where you can be the big fish. 
These Unicorn Fish are a fictitious metaphor for "Unicorn" startups. I intend no relation with what marine biologists call Unicorn Fish. I discovered they're real after writing the essay. Although I'm willing to bet that they actually evolved in a pond-like environment, due to a more generalized premise in biology (which I've argued for startups here) called Allopatric Speciation: https://en.m.wikipedia.org/wiki/Allopatric_speciation
The above dealt with new markets. In existing well-understood markets, the same strategy applies. Marketers call it "beachhead strategy". Beachhead is a metaphor for Normandy Beach, France, where the allies concentrated their forces before invading a Nazi-occupied France and liberating Europe. To liberate customers from an incumbent, you must largely focus on a small beachhead.