That friend who always organizes social gatherings - they pick the restaurant, get a headcount, pick a time, make reservations, etc. "Top" here is a reference to the dominant role during sex, especially in the gay community. A social top is dominant in a social context.
I'm glad you are back from vacation - without our social top, we just sat around at home and wondered what everyone was up to.
by sneeper October 04, 2013