The following would lead me to believe that the original Phi Beta Kappa was a social fraternity...
"Phi Beta Kappa, the nation's oldest and largest academic honor society, was founded on
Dec. 5, 1776, by five students at the College of William & Mary in Williamsburg, Va. It was the first society to have a Greek letter name. And in its early years it introduced the essential characteristics of such societies: an oath of secrecy (discarded in 1831), a badge, mottoes in Latin and Greek, a code of laws, and an elaborate initiation ritual.
The members held regular meetings, generally with an emphasis on literary exercises, especially composition and debating. Fraternal sentiments were fostered, and
the purpose of some meetings was simply good fellowship. Anniversaries were celebrated in the Apollo Room of the Raleigh Tavern, which the College of William & Mary has recreated on its campus in Phi Beta Kappa Hall."
For more:
http://www.pbk.org/about/history.htm