Most startups are not just built for the person who is using them. When you do that, every now and then you get really lucky and... are representative of some huge class of people who all want the same thing you do... but very often that just turns into a side project that doesn't go anywhere.

The pool of people you care about is going to shift over time.

This is true for most new products. The majority of people you're competing with are non-users. They are people who have never used your service before. And what they say is actually the most important. What they say is the thing that blocks you from expanding the size of your market with your features.

The other mistake I think people make is talking to who's available rather than talking to who they need to talk to.

I wish I could tell you the recipe for figuring out who the target user is for your product and who your users should be, but... there isn't a recipe. It comes down to think really hard and use your judgement to figure out who you're really building this for.

A goal of Twitch is to be wherever gamers are, whether its on laptops and handheld devices or integrated into gaming consoles and software.

Who would you talk to? That is the first question for almost any startup that you need to answer. Who is my user and where am I going to find them?