Fr. Z asks: How Large is the 'fallen away Catholic' denomination?

The comments section has some interesting thoughts on why people fall away from the Catholic Church, an area of interest of mine. I will list some of the things mentioned there:

the Church's 'rules' on divorce and re-marriage

Touchy-feely watered down liturgy, with touchy-feely false teaching got us into this mess

Poor liturgy, apathetic laity, priests that don’t understand their own ordained powers, poor formation, the constant battering of the liberal media, political correctness, even fear of being open about our beloved Faith

sense of sin has evaporated, nothing matters, ‘everybody is right, you are a good person, I’m a good person’

It wasn’t apathy or the lure of the outside world that drove us away from the Church, it was the behavior and the attitude of the clergy and the religious themselves.

The #1 reason may be the overwhelming sense that God is missing in the life of most parishes/Catholic schools.

Lack of attention to transcendence

The comments section has some good thoughts on how to get people to return, some of which may be right or wrong, but the important thing is that Catholics (especially priests) make the attempt. I think the cleansing of the priesthood that really began in 2002 and hopefully is continuing, will help, as well as the reforming of seminaries. The religious communities that are faithful also can inspire Catholics to deepen their faith and practice. Although many people still try to deny it, orthodox thinking, preaching and living is what attracts people and there is plenty of proof of this for those who have ears and eyes.

And to answer the original post's question, inactive or 'former' Catholics make up the second largest 'denomination' in the USA, right behind Catholics.