Trident

LIBBY: You know, they say four out of five debs marry their escorts.
RORY: Kind of like the dentists with Trident.

Trident is a brand of sugar-free chewing gum. For many years it was advertised with the slogan, “Four out of five dentists surveyed recommend sugarless gum for their patients who chew gum.” This is believed to have been based on a survey of practising dentists in the early 1960s, the results of which were approximately 80% in favour of sugarless gum.

In real life, it’s not true that 80% of debutantes marry their escorts. That’s preposterous, especially as most debutantes are matched up with some random guy. I’m not sure whether it’s meant to show Libby is a gullible fool, or whether the show actually expects us to believe this nonsense.