Until I read this thread, I didn't realise that this had "Happened to me!" but I have a somewhat similar story...
I belong to a group of six or so friends, who by association, are connected to another group of friends of an equal number. We see each other at parties, but don't mix such a lot beyond the usual pleasantries. Even after meeting this other group on several occasions, my partner and our mutual friend didn't know their names terribly well.
At one of these parties we were told that "Ralph" would be joining us after work. It got late and the three of us (as we discussed after the event) wondered where the person we thought was Ralph was. Later in the evening someone addressed one of the chaps already there as "Ralph", much to our surprise! On the way home the three of us admitted that we hadn't realised that was Ralph and that we all felt there was someone missing, someone we had mistakenly assumed was Ralph at a previous gathering. So, who were we all waiting for?
We've been to numerous parties with these other people since, and even discussed our confusion with the larger group, but there is no other friend who is unaccounted for, no acquaintance who only appeared the once. There really wasn't anyone else it could have been, because the Ralph we imagined was an integral figure in the group, not a hanger-on, but a close friend of those present! I honestly remember seeing him pop up on social media and the like!
We've taken to calling this missing person "Other Ralph" and have been quite amused by the whole situation. Until today, that is! After reading this thread I realise that my feelings mirror the OP's quite closely! I feel confused and disoriented by the whole experience. After finding this thread I sent a text to my (luckily!) Fortean friend, who is at the center of the "Other Ralph" phenomena, and whilst he doesn't feel that it's anything paranormal, he admits that he has trouble thinking about or concentrating on Other Ralph and that described my experience exactly! It's like something I can't quite grasp, something intangible. I don't know. It really does feel like someone blinked out of existence. I feel like I knew what this person looked like, but that the memory's disappeared somehow! The oddest thing.
I realise this is easily explained away as social confusion amongst a group of people I didn't know particularly well (which I'm sure is the actual explanation). But for three of us all to have a vague awareness of Other Ralph strikes me as slightly odd, especially coupled with the slightly dizzying emotions that go along with it, after reading this thread!
Make of it what you will! Just thought I'd share as the case seemed somewhat similar! Other Ralph will live on forever in our hearts!