If I'm introduced to you, and we talk for half an hour, I will not recognize you an hour later. If you remind me who you are, then of course I'll remember the conversation and your name and everything, but I just have zero visual memory of people.
This is even to the extent that my wife once sat down next to me on the train (we didn't usually make the same train), and I spent the entire trip thinking "I think I know this woman from somewhere, maybe?" before she finally couldn't hold it in any more and told me who she was.
(Yes, I score in the bottom 5% of prosopagnosia tests.)