There once was a plattform, think it started in Munich called "die Lokalisten"
You only could join by a invitation from a real life friend. And than you saw .. ah that Person was invited by ... and meeting a new online person you saw all the real people between both. I loved it because you Couldnt fake!
But than they got greedy. Easy open access. Lot advertising. And finally sold for a lot of money to a big Company endet up as a - in my eyes - unfriendly bunch of liars and fake daters.
It was a great concept with a "proof of existance" wich had lot of positive impact on peoples behavior
When Google started Gmail, it was invite only - it was a very easy way to get a lot of people in, because it makes it look "scarce" - once there are enough though, there is no point. The critical mass is reached, people can get an invite from anywhere anyway.
We are easily hacked.