I think everybody has such a friend: you are not really close friends, you mainly talk about one topic. This topic is the reason why you text/message your friend and mostly not things like "how are you?".
Here's specific example:
I've got a friend who lives 9000 km away with a time difference of nine hours. That's why it's pretty hard to reach her and it's nearly impossible to talk to her everyday to get the latest news. I already visit her, so she's more than "someone that I used to know". But we share interest in one topic, so everytime she or I see something about it (e.g. book, ad, social media post) we're sending a text or a photo. But that's pretty much it, we don't ask for work or the daily routine, because we noticed it takes too much time till we get the responds, e.g. if she asks "how was your day?" after she's done with work, I'm sleeping so she'll get the responds hours later and it has nothing to do with my mood the day before.
I think we all have such friends, you meet them once or twice but it's somehow connected to a topic (maybe a band or a movie).
So I am asking what would you call such a friendship, if it's a friendship after all.
I was thinking about a sketchy friendship maybe? I'm sure you'll find better words.