When you hear the name of a person, is your first reaction "who?" or "them"? This instinct is a good proxy for how famous that person is to you. It's common to have these types of conversations about musicians, actors, classmates, and co-workers.
Reflecting on how often I talk about "who"s versus "them"s is an interesting way to assess how much I have in common with someone. A high ratio of "them" to "who" suggests that we have shared experiences or interests. And when the ratio is low, we need to make a concerted effort to find common ground or the relationship may not be worth investing in.
If you're looking for a way to increase the "who" to "them" ratio in a conversation, you may want to consider talking about sports or politics, since they always have new developments.