Engagement for me as well. Now, I can't speak about what is best for earning revenue or building your fanbase, etc because I am still a newborn baby when it comes to all that.
But what keeps me looking excitedly at my notifications every day? What keeps me updating? ENGAGEMENT. Texting back and forth with a friend who likes my novel or comic, or answering comments on tapas, or seeing stuff on my wall, or even getting into an interesting discussion in the forums, is why I am genuinely enjoying the experience and excited to keep doing it. It's why I finally achieved my goals of uploading many formerly "lost" comics of mine onto the internet once more where they can finally live and grow. It's why I FINALLY wrote my novel.
The people, even if it's one or two, and how they read my work, is why I finally got off my ass.