incoming freshman

This isn't really a good question since literally every sorority is social with someone. What makes the tiers is who they socialize WITH. so just look at the rank posts if you want that.

every sorority has mixers with most of the frats (exception of the very bottom tier)
as long as you dont go bottom, youll have plenty of social oppurtunities

Sigma/Zeta - tied for most fun/social. Depends what girls and what guys you ask.
Sigma Kappa - don't consider them top tier yet, but these girls are out a lot at a variety of parties and all pretty crazy/fun
Tri Delta - lower than SK but again, more fun/out at a mix of frats - underrated in my opinion
Phi mu - still ranked relatively high since they're ranked high. Very exclusive though and hang out with the same groups of guys.
AxiD - haven't been seeing these girls out as often as I used to, but these girls are social for the most part.
AOII - some go out a lot and are super fun, others don't
KD/AXO - can't comment because I don't really see them out ever