Difference is that email doesn't have any network lock-in. If I'm the only one with a gmail.com account I'm the cool kid who can email everyone still on hotmail and yahoo to make them jealous, and now they want an invite as well. If I'm the only one with a bluesky account then I'm just...shouting into the void and no one cares.

Facebook was initially .edu addresses only. It created a sense of exclusivity that had regular people desperate to join.

That exclusivity was a feature, not a beta test. Facebook was a college-only social network, and operated that way for years.