IRC isn't federated in that sense, users on EFnet can't interact with users on freenode, so the network effect works in that case. Gmail has an enormous market

IRC isn't federated in that sense, users on EFnet can't interact with users on freenode, so the network effect works in that case. Gmail has an enormous market share (about 40%) but I don't think that's related to the nature of SMTP, more about their incredibly competitive free tier and the decidedly not federated groupware for their business offering.