Yep, it happens a lot at my school too (even though we don't have sorority houses), but I agree with honeychile that it shifts. A few years ago we dated/ hung out with mostly Kappa Sigs and swapped with Kappa Alpha Psis, and now we mostly date/ hang out with TKEs and SAEs. Go figure.
(No ZBTs at our school
, but my brother pledged ZBT at his!)