Challenge: Getting an initial audience before official launch; How to overcome it?

Say you made Instagram. Your first competitor is Facebook. How do you get the initial audience? If I were a new user and saw Instagram - I would not engage on the site unless it has a large of audience already (eg; private networking where you have a beta test and somehow gathered thousands of users before launch) . Otherwise, what's the point of creating a social media account when no one uses it?

I believe a lot of startups come across this challenge (it does not have to be related to Social Media, such as Youtube, Twitch, eBay - I believe all of them requires an initial audience first before others want to engage on it). How did you guys overcome it?