I had a really interesting epiphany regarding this aspect of MMOs just several months ago, while playing Throne and Liberty.
I had joined a guild in one of the top alliances on the server. We were one of the first to win siege on offense, outnumbered, etc. Well, I started noticing more and more that basically all of our "social" interaction was via Discord. The guild chat, alliance chat, etc. was practically dead almost every day. You'd have thought there were 5 people in the guild at any given time. Virtually EVERY bit of coordination, events, planning, shooting the shit, etc. happened via Discord. Guys would even log in on their phones on their lunch breaks at work just to chat and socialize.
It was really that watershed moment for me where I realized that socialization via the MMO itself was all but dead. Hell, even joining the guild was done via a Discord link that went to their charter, some brief Q&A, messages to the leadership, etc. The game was responsible for virtually zero of the social aspects.