As far as I know, there’s no real definition in terms of size, but in general, a micro-influencer is someone with a small audience of highly targetted people – small enough that makes it uneconomical for big brands to work with them, but targetted enough that it’s very economical for small brands.
There are a few platforms around that exist to enable these relationships, I don’t have any direct experience so I’m reaching the limit of my usefulness on this topic but I know Tribe is a popular platform and their page for creators gives you an idea of what you’d pay per post.
That’s a really interesting idea: writing as a Kinopio use-case. I’ve used it a little for that and found it helpful as a way to get ideas fleshed out and organised before I started writing but I just used Kinopio because it’s where I put my thoughts 
There have been some successful blog posts and books on the topic (e.g: Write Useful Books) so I think the demand is there, and Scapple clearly validates that the Kinopio model is good for writing…
…yeah, I’m convinced, I think “kinopio as a tool for writing” could resonate, get people in the door and realise the value. I’ve been sitting on a few blog post ideas recently but struggled to realise them, you’ve inspired me to see if I can bring my vision for these posts to life using Kinopio more intentionally as a tool for writing – which I… hadn’t considered before.