It's usually twitter, I believe. it's a bit different than in English because it's very easy and efficient to communicate via twitter in Japanese, so many people use it as a friend base. Also if you've just started as an utaite or something and you don't even leave a ping on their radar then I'm sorry to say but they probably won't become your friend. There are too many regular fans trying to do that, and you'd just become one of them. You can talk to them, of course, but it'll be hard for them to remember you as an individual.