[Silhouette]

They all fail to reach critical mass until someone does. That's always been the history of social networks. Once sites like Myspace and LiveJournal were everywhere. The next generation mostly went on Facebook and Facebook snapped up Instagram. Now a younger generation is on sites like TikTok. Digg and Slashdot are still going but they didn't stop Reddit becoming huge or more specialised sites with overlapping demographics (like HN for example) from building their own communities.

You're right that early movers have some advantage but it's a big world and the Next Big Thing doesn't have to win the whole market on day one - only enough of it to plant seeds that can grow over time.