For some people, the answer to the question asked in the header is very simple: “I don’t.” And that’s totally valid. Some people make friends in a game and then treat those friendships as ultimately temporary. We’re both here doing a thing for now and having fun, when we stop doing the thing we stop being friends. It’s nothing personal, it’s just not meant to be a fully rounded friendship.
Other people do not subscribe to that philosophy. After all, you already know you share at least one common interest, and you might share others! Sure, it means forging a real friendship that may very well run afoul of problems like the simple nature of where people live relative to one another, but who hasn’t occasionally made friends with seven people who are all on different points on the globe so that none of you can see one another but you can all want to all the time? It’s delightful!
Ahem. The point here is about methods. So how do you stay in touch with your MMO friends outside of the game? Email? Text? Discord? Smoke signals? Or do you just… not?