I think this largely depends on the type of campus (i.e. state, private, religious, etc.) as well as the size.
I graduated from a small, private, Lutheran school. We had 3 social local sororities, 1 national service sorority (Omega Phi Alpha), 3 local social fraternities, 1 national professional fraternity (Phi Mu Alpha Sinfonia), and 2 national co-ed fraternities (Alpha Phi Omega + Phi Beta).
A very small percentage of our student body was greek (and the entire body was under 2,000 when I was there).
I don't think I can give you an average for schools in general, as I have only attended one as an undergrad
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