The Southern Baptist Convention, the largest Protestant Denomination in the United States, reported 16,287,494 members in 2004. However, only 6,024,289 people—or 37 percent of the membership total—attended church per week (on Sunday morning or whenever the church had its main worship hour). This is according to the Strategic Information and Planning department of the Sunday School Board, 2004 statistics.
The Assemblies of God, another Bible-believing denomination (though of a Pentecostal persuasion) called the 1990s the "Decade of Harvest." Of the 3.5 million people reported to be converted to Christ in that time, only 5 percent actually began attending church.
Reference: Jim Elliff, "Southern Baptists, an Unregenerate Denomination," Christian Communicators Worldwide, CCWonline.org (2005)