We are a world of first names.

"Mark, this is Tara, Brittany's roommate."
"Who's Brittany?"
"Oh, maybe you don't know Brittany. Okay, well then this is just Tara."
"Tara, this is Mike."
"Morgan, this is Lore; you remember her?"
"Beca, this is Jackie, and this is Morgan."
"Girls, this is my friend Annie, like the orphan. Only she's not. An orphan, I mean. Just Annie."

No last names are ever exchanged, unless it is for the purpose of clarification:

"Kinworthy."
"Kinsworthy?"
"No. KIN-Worthy."
"Kinsworth?"
"Avnaim, like Nick and Tony, she's their sister."
"Oh, Baker! Mike Baker, yeah, I know who he is."
"Ferguson! What kind of crazy name is that?"
"Well, it's mine for starters." "
Oh."
"The mailbox says Philips Morris, like the cigarette guru, only it really is just our last names."

We are a world of first names. Which is why, incidentally, I've finally begun programming last names into my cell phone alongside first names. I want to know some of these people in ten years and don't want the reason for lost contact to be that we were too lazy to acquaint first names with last names.