The promise of future value from a captive audience. The idea that no competitor could succeed because everything worthwhile is already on Twitter. A self-reinforcing network effect.

IMO, this sort of held up, even with Musk's hijinks. There has been no mass migration to Bluesky, Mastodon, or Threads. The problem is, people are just moving shortform expression to video-based networks, like TikTok or Instagram.