To be honest, I am a fanfic writer. So I have this advantage of almost delivering a service to my readers. I write for the fandoms they love. In exchange, I get such a load of love and emotional support that it is frightening. I get more affection and commitment from the readers of my fics than from my friends. I guess, it has to do with the fact commitment goes both ways in the fanfiction realm.
What generally really works well for me, but is also hugely time-consuming, is every time I get a follow, favorite or review, to go into the person's personal messages and write them a personalized, thought-out message. I tell them about how much their support means to me. About how it made my day, if my day was bad. About how it helped me dissipate bad feelings or what the characters mean to me, what sides of them I try to explore or put an emphasis on.
By writing about myself, without making it all about myself, the readers see that I am not an automatized screen that just poops out updates. I am a human being with my ups and downs, with a passion … Just like them. Sure, like 70% of the readers don't care about me and never reply back. But the 30% there … those are pure gold. Some of my best friends I met writing fanfic and have been in touch with for over eight years now.
Don't make it about reviews, subscribes, make it about the other person as a human, look at what their art tells you about them, write them thought-out reviews on their work that show how you engage with it. And be sure that when and if they leave Tapas, they will take you with them (whenever I left somewhere, I left emails, Skype IDs, Facebook accounts behind for them to be able to reach me … and anyways, leaving never was an individual decision, the friends I made writing were also allowed to express themselves and were active players in my life, just like my IRL friends).