Instacart's first customer was its founder. Then a few of his friends started using the app, because for them, as much as for the founder, Apoorva Mehta, shopping for groceries was a big pain in their lives.

The app recently closed $44 million in funding.

Narrow down your market when you're building a product."Sometimes the trick is to focus on a deliberately narrow market,"  Y Combinator co-founder Paul Graham writes "It's like keeping a fire contained at first to get it really hot before adding more logs."

To give you some examples from existing successful products, Facebook was launched only for Harvard students. Uber was launched for customers only in San Francisco, as was Instacart. And even Starbucks started with just one store in Seattle.

Look where they are today. They all built a product that only a few people wanted, but so desperately that those customers couldn't imagine a day without its existence. So passionate were those people about the product or service that they made sure the entrepreneurs were able to scale to a much larger audience that had similar problems or pain points.

How, you ask? Here's how building only for a small audience that desperately needs a solution can help your business become successful.

1. Making mistakes

The only validation for your product is when customers use it consistently. Getting your product in the hands of your customers at the earliest possible time then becomes imperative to its success.

When you identify and build for a small number of users, you get the opportunity to make your early mistakes with only that select audience. You get to learn fast from those mistakes to continue iterating on the product up to a point where it resonates with that initial audience.

Your mistakes are contained, your learning is fast, and the quality of feedback will be far superior, as you can observe user behavior firsthand.

Once you hit the target of your first 100 users, connect with them personally as much as you can. That means picking up the phone and speaking to them. It also means driving out to meet them and observing their behavior while they use your product.

The more you interact with them, the more feedback you'll get on their needs and whether your product is working in its current avatar or not.

For example, if you're making a product for fixing playdates for children, and assuming parents will send their kids to someone else's place without first interacting with them, you can learn quickly whether you need to build some security measures by signing up a few users (100 or 1,000).

2. Minimizing costs

Trying to reach out to your audience nationally or globally on day one would essentially mean you launch a marketing blitz costing several thousand dollars, at the very least.

All this, without knowing whether even a small subset of this audience would want to use your product.

The benefit of building for a small audience is that you will save these thousands of dollars and incur costs of only a few hundred dollars (at the most) in recruiting customers manually.

Llet's take the same playdate concept as an example. By going to your local schools in the vicinity and talking to parents there, you can acquire your first set of customers manually without having to spend money on marketing. You can gradually work your way through different schools in your city once you've identified a product-market fit with the current audience.

3. Influencing word of mouth

The more you interact with your initial set of users, the more they engage with your product and brand. The more they're engaged and know that you're interested in building a solution for them that works, they more likely they are to continue using your product.

Once you've hooked a user, that user becomes an evangelist for your product or brand. These first sets of passionate users start spreading the word about your product among their network of friends, family, and colleagues. Make sure you provide enough tools for the users of your product to help you spread the word about it when they want to.

For example, if you bring a friend of yours onto Uber, both you and your friend get a credit when the app is used. Now, first, you would recommend it to your friend only if you had used it at least once and had a great experience. And once you've decided to recommend the app to your friends, Uber makes it easier for you to do so and at the same time delights you by incentivizing.

This is how some of the most successful apps got traction--not by the companies spending millions on marketing but by investing their time in delighting the first few sets of customers, to the point they couldn't help spreading the word, triggering a viral effect.