The sad fact is that lots of startups don't know how to acquire customers. You'd also be surprised at how many assume they don't need to 'acquire' users - they subscribe to the field of dreams way of thinking - 'build it and they will come.'
I've lost count of the number of amazing products I've found that are incredibly useful and yet have a tiny userbase. In too many cases, it's because they haven't done any media or user outreach. In many cases, they have an overcomplicated onboarding process. In a lot of cases, they don't have simple social hooks baked in to encourage people to invite their friends.
Why does this happen - in many cases, companies don't have a strong marketing person on board early on. Those are the people who will drive that initial stage of customer acquisition. They're the ones that will be going out, tweeting, connecting with journalists, bloggers, potential partners and anyone who might be interested to get them to write about the service, suggest it to their friends or just try it out themselves.
A lot of the time, people aim for moonshots instead of doing simple, effective things to launch a product. I've met a lot of companies who have pinned everything on a massive partnership with a global brand or outlet without ever looking at something smaller or local as a test case.
You'd be amazed how easy it is to get media coverage in papers, radio, TV and on blogs with just some simple approaches. You'd be surprised how easy it is to find potential users and partners on social media and other channels with some simple searches. It should be easy to get to several hundred or several thousand users with these simple approaches.
To scale from there - simple things like a really easy onboarding process, social hooks (invite your friends, rewards for getting other people on board etc) and a level of activity that encourages repeat visits is massively important. To give you an example - the reason that you need to follow about 8/10 people when you sign up for a Twitter account now is that they lost lots of people after they signed up, didn't follow anyone and then logged back in to find an empty screen. Once you follow people who tweet regularly, you'll have fresh, interesting content when you go back. Having that freshness or activity is really important - social proof and little bits of feedback matter to people.
I'm happy to talk through some more specific examples of where people have gone wrong and how you can do simple things to help acquire users if you're interested. Feel free to get in touch.