Facebook was never exactly "invite only" but when they started it was limited to certain universities, and they expanded gradually. If a social network is only going to have a few user it can still provide value if they all have something in common; better 1,000 users all in the same city/university than 10,000 users scattered across the globe. Threads doesn't have this problem since they can just do a full-court-press immediately and get 30M users in a few days. I dunno how bluesky works but if they are reasonably generous with the invites you can get your friends on it (who are interested) pretty quickly.