Let's not muddy the waters more. The question was which is the
largest women's organization, not which is the
top organization. It's objective versus subjective. The list you provided, while interesting, was clearly a list of which orgs the "womenissues.about.com" website thinks are the "best" orgs for women's issues. (Notice a fairly uniform perspective reflected among the groups?) Note that some of the orgs listed, including NOW and the YWCA have female and male members. And according the website of the League of Women Voters, that group has about 130,000 members -- quite a few less than the numbers posted here for many sororities.
Yet another reason why this thread is a dead-end road.
Hybrid Mode
