In his essay titled "Do Things That Don't Scale" Paul Graham talks how in the initial stages of a startup, it can be really hard to gain traction. That’s why you shouldn’t wait for users to come to you but try aggressively to recruit them. This idea might not appeal to the engineer mindset as it might appear to be not scalable or sustainable. But even small gains at a very early stage can add up to significant numbers in the long run. Besides you don’t have anything to lose at an early stage but have a tremendous payoff. Also the fact that it is not scalable means that it is to your advantage to do this since big companies can’t compete with this approach. Paying strong attention to your existing users can gain you customers for life.
To find users to recruit manually, you might want to create something that solves your own problem. Then you can recruit your peers as users. Or you can do a comparatively untargeted launch and try to see who is more enthusiastic about your product. This happened with Pinterest when they noticed there was a strong design interest for their product. This is not only about finding and acquiring users though, but also making them really happy. The feedback you will get from engaging with your early users will be the best you will ever get.
Couple of things to keep in mind that can be derived from this rule. If there is a subset of people in your target group that you can target to get a critical mass of users quickly, then you might want to do so. This will help keep things a bit more focused to begin with (like if was for Facebook when it was first launched) and also the subgroup will feel special because your product will appear targeted to them.
You can run a business that is based on automation manually, until you can’t. This can allow you to launch a product early and acquire users that way.
That’s what’s meant by ‘do things that don’t scale’. These things fly on the face of efficiency but will get you leverage in the short run. But if you are to get bogged down by the concerns of scalability when you are only starting out then you might miss out on opportunities of growth and sources of motivation like sense of delivering value to people which might impede and even ruin your progress.
You should really check out the original essay though, it is full of amazing advice for startups at their early stage.