I'm a millennial and I use Instagram (mainly stories) as the primary tool for keeping in touch with people. It's easier to share an image, maybe with a short caption, rather than composing a post, and consuming friend's stories are quicker than text posts since you don't need to read. It's a more efficient/faster way to know what my friends and family are up to and it prompts me to communicate with them more.
I may be an outlier but I'm very bad at remembering to text/call/email friends if I don't see them regularly, so following them on IG helps a lot.