As of right now, CM has signed up 2,397,750 members (this number will change before I can post this answer). How many are active I couldn't say. There have been 112,166 new groups created. Based on media interviews I have seen the number of actually active groups put anywhere between 35, 000 and 65, 000.
As the #1 media property for moms, CafeMom reaches 6.7MM unique visitors per month on cafemom.com (comscore, May 2010) and 18MM+ unique visitors per month on CafeMom Plus (comscore, May 2010),
from CM's 'About CafeMom' section.