In the UK, it appears to be illegal to even develop a website or app that "instigates" a user to invite their friends to that website or app. This includes pre-filling an email that they send their friends. So, any sort of viral mechanism that people on YCombinator and Hacker News have used to grow their websites with "hockey stick growth" is illegal in the UK since 2020:

https://dpnetwork.org.uk/opinion/refer-a-friend-viral-market...

“An online retailer operates a ‘refer a friend’ scheme where individuals are given 10% off their orders if they participate. The individual provides their own name and email address and the retailer automatically generates an email containing its marketing for the individual to send to their friends and family. The retailer is instigating the direct marketing therefore they have responsibility for complying with the PECR rules. Because the retailer does not have the consent of the friends and family these emails breach PECR.”

That's right, the UK is policing what businesses can provide for people to privately send their own friends! There is simply no easy way for your users to legally invite their own friends to your site, so it can never reach critical mass. This would probably destroy the business model of 99% of YCombinator's B2C companies, Facebook, Twitter, and any website with "invite a friend to do X with you" functionality.