For many a celebrity, it's basically a given that every new face they meet knows who they are. Knows them, that is, in some peripheral, Page Six headline, magazine cover way. No you didn't go to high school together, but God knows you wouldn't need to think twice if asked to name their last 3 boyfriends.
They've accepted that you know everything external about them. Now you have to. Knowing of someone isn't all that awkward - you just have to be sure not to assume a level of familiarity with them. Jumping from hellos to asking them how things are going since that naked photo scandal, not cool. However, a lighthearted "Did I hear that you just had your second child?" or "I have to say, your last film was so so brilliant," are totally within the realm of acceptability.
[Photo via @derekblasberg]