Friendship with only one topic

Question

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.

Answer 1

In U.S. English, it's common to refer to people with whom you have a positive relationship based on a single shared activity by the name of that activity followed by friend or friends. For example:

• He's a volleyball friend.

• She's a birdwatching friend.

• They're writing-group friends.

• We're drinking buddies.

I don't think that a widely adopted generic term for such friendships exists. None of the possibilities that come to my mind—"single-connection friends," "one-activity friends," "shared-interest friends," "context-dependent friends," "facet friends"—seems especially appealing.

Answer 2

May not be the exact answer, but I simply think of something that I never heard before, "topical friendship". And I've found something, that in my view, support the idea.

There’s nothing wrong with Carole or Tanya for maintaining this mostly topical friendship that only occurs when they happen to see each other. Just think about how often a grocery store cashier asks, “How are you?” before s/he starts ringing up your purchase. Your response is probably an automatic “Fine.” I’ll bet you don’t dump your family drama, a recent health scare and financial troubles on the conveyor belt along with cantaloupe and dish detergent.

Update:

Inspiring with @Edwin's comment and based on the page 470 of "The Global Internet Economy", it seems that Homophily could fit the situation that OP describes. In Wikipedia it is defined as:

Homophily (i.e., "love of the same") is the tendency of individuals to associate and bond with similar others. The presence of homophily has been discovered in a vast array of network studies. More than 100 studies that have observed homophily in some form or another and they establish that similarity breeds connection.These include age, gender, class, and organizational role.

Answer 3

This is a casual friendship. (As opposed to a close friendship.)