While your assumption that you do have or will have dementia may be accurate, that would not be the first conclusion that I would draw from such an episode.
That is, if Facebook is a casual application for you, or if you post frequently there without writing essays or without a great deal of emotional attachment to each post, then it’s like a lot of other casual things. Do you recall all of the meals that you had for lunch and dinner last December? I can barely recall the ones I had last week; they’re casual things without a lot of emotional attachment.
However, I can recall pretty well the few times that I went out to eat with friends in the past few months, because I don’t do that very often, and I enjoy my friends’ company when I do. (However, I still can’t easily recall everything we talked about – only a few things.)
I make a lot of casual posts to Facebook that garner “likes” weeks and months after I made them. Sometimes when I’m reviewing my notifications and see a “like” of a post that I made long ago (since FB doesn’t say in the notification the date of your post that is being reacted to), the partial comment that comes up on the notification doesn’t remind me at all of what the comment or the topic was. In fact, sometimes I’ll click on the notification to go back into FB to see “What is this about?” and I’m often not fully reminded until I’m back in the thread and reviewing the top post that’s being commented upon. (Same in this forum, too, sometimes.)
And dementia does run in my family. My mother had pretty much lost most of her memory for events and things toward the end of her life (although she still had a good memory for family and friends), but I’m not worried. Maybe I’m just too far gone to be worried, and I should worry about that… but I’m still not worried.