You never explained it to begin with, so you didn't do it again. Thanks for clarifying. There will be plenty (spammers, stalkers, attackers, bots) who will send 21 sats for friend requests and the advertisements will be in the profile names and profile pictures. Also will be a lot of fakes, attempting to infiltrate friends lists. Many will succeed. Many will loose their private keys to scammers. Those people will "call the support center". Did you know that a large part of Facebooks ad revenue is from scammers? Mitigating these things costs a lot of money. Nobody is willing to pay the actual cost of the services they use. Everything is subsidized by VC money or ads. Exactly what you want has already been attempted, "Lotus" it was called. The only difference was that you could set the cost of your inbox ourself, which would be better than a fixed 21 sats. The concept is that if you are attractive for some reason and people want your attention, they will pay more. You can also make it 1 million sats, but you "pay it forward" or "pay it back", depending on the scheme. So your family already knows you, so they don't have a problem sending 1 million sats in a friend request to you, they know they will get it back. This exact concept has already been done in traditional social media apps, I don't believe it had any long term success.