True - but the Catholic Church doesn't recognize it's own (self-)excommunications as valid when it comes time to count membership. How else could there be 68+ million Catholics living in the United States right now?
As Wuli said in post 13, "somebody didn't get the memo".
You raise an interesting question: how are Catholics (or other religious believers) counted in a census?
Each Catholic parish keeps permanent documentation of baptisms, confirmations, marriages, and burials. This does not amount to a "membership list" for the parish, since obviously most people are not baptized, confirmed, married and buried at the same parish, the same diocese, or even the same country.
A parish census is, as far as I know, no more than a print-out of who's on the parish mailing list, i.e. who gets sent a notice that Council of Catholic Women is having a bake sale to support the Pregnancy Aid Center, the Parish Nurse is reminding everybody to get a flu shot, etc.
My husband has been getting these mailings for years, even though he is not a Catholic; and I have NOT been getting them although I have been a member of the parish for 19 years--- an omission probably explained by the fact that I make my weekly donations by cash and not by check.
As to the matter of excommunication latae sentenciae -- one incurs automatic excommunication, for example, by procuring an abortion, but nobody has ever polled the parish to find out who has had an abortion, nor is there a system of surveillance and informers, so how would anyone know?
Like everything else in the Church, the whole thing is based on voluntary compliance. In fact, it always amazes me that some people think the Catholic Church is a hyper-legalistic institution, when in fact it is run on the basis of reasonably efficient anarchy.
Private polling does little better, since Gallup and Zogby and the New York Times typically ask people what religion they "identify with," or what religion "they were raised in," which doesn't take account of people who were "raised" as Jehovah's Witnesses but are now daily communicants at St. Leo's, or who "identify with" Catholicism because their name is Murphy (!)
Last week the big Pew Rsearch Center survey of 35,000 adults showed that over 1 in 4 had left the religion they were raised in, and switched to a different church. Over half of the switchers had switched more than once.
I don't think any of the membership census data show the complexity of people's religion affiliation, let alone their level of belief and practice.