How to build an audience online

When I think of an audience my immediate vision is a crowd at a rock concert or an assembly for a speech or play.  My second thought is an image of a family gathered around their television or if I get historical, their radio. As I loosen the boundaries of my thinking the idea of what an audience is starts to rapidly expand.  Today an audience can be the readers of book or blog, listeners of a podcast, people who watch a Youtube video, visitors to a website, subscribers to an email list, users of an app, people that share, like or comment in social media, the list grows and grows.