I don't have an specific story, but college has become something like that for me.
The students have independent schedules, and there's no guarantee that you are gonna have the same teacher, teaching the same subject, in the same classroom at the same hour as a friend of yours. Also, we have a limit among how many times we can fail a subject, meaning that, if you failed three times the same subject, you get spelled no matter if you ar A-OK in all the other subjects.
While I can count three persons, my detached behavior, combined with the aforement situation has made me have a lot of that moments every single semester, and in the luckiest cases, I can maybe remember you and become this. At worst, you practically dissapeared from my life , even if we worked together in a project on previus semesters.
Among the ones I can remember their name are Ramón, M Isael and Greta, to name a few.
As my profile page says I have had several people I knew on the internet disappear without a trace. One I feel really sad about