In ranked order of number of posts in their forum:
Sororities
AKA- 113,940
DST- 106,085
ADPi- 23,733
AGD- 16,345
ZphiB- 14,812
SGR-14,668
Alpha Phi- 7,814
SK-6,339
DG- 5,367
ZTA- 3,856
AXO- 954 posts
Fraternities
Alpha Phi Alpha 13,696
KAPsi- 5,852
Omega Psi Phi- 6,403
LXA-8,892
Yes I did leave a lot off, but the major posters are here. I listed a few like AXO because they were mentioned in this thread.
Well SK is up there for the NPC, but we're WAY WAY far behind in sororities.