I always reply to every comment I get and I upvote it. I mean, my upvote is small, but at least I show some form of appreciation to my followers. Engagement is important and I try to do that. As for sharing your posts on other social media, I think it's a great idea and it never came to my mind. I guess more people should be doing it to have more interaction on this platform. I've noticed some people with bigger audiencies on other platforms never even notify their followers that they are also posting on Steemit.