It's acceptable and encouraged to follow unknown people on IG. I follow people with hobbies I like, some of them become friends due to interaction. Unfortunately this works only for hobbies that can be fit in a photo, I'd like to have a more generalized platform (anyone remembers Livejournal? :)

If you read comments here, you will find a few people who lost their significant others, because they got too many friends from Instagram and irreversibly changed!