Graduation scatters everyone. The friendships that make it are the ones somebody decides to keep feeding.

Everybody scatters and nobody plans for it, because the friendships felt structural back when you saw each other by default. Once the default goes, staying in touch becomes a job somebody has to pick up, and usually nobody does. The ones that last are not the deepest, they are the ones where a person decided to be the organizer and kept going through the awkward early silences. Unglamorous as that is, it is the entire mechanism.

Decide now which few you want and take the job for those. Put a recurring call in the calendar and be the one who messages first. Expect to carry it for a while.