An email-powered social network that could disrupt Facebook and LinkedIn

Ironically this is almost exactly how it was done in the 90's. Listserv is patented apparently, but there were a few different discussion list methods available at the time. They were all pretty similar in operation, and the differences mostly amounted to maintenance. If I recall correctly, Smail had a super-easy way to manage discussion lists.

https://en.wikipedia.org/wiki/Electronic_mailing_list

Several lists still exist. They're just not as common today as they used to be. I subscribe to some mailing lists for Lisp, NetLOGO and Artificial Life - all subjects that were pretty typical topic areas for 90's mail lists.