Based on a recent 2009 report of church membership in North America by the National Council of Churches, NCC, JW's are near the bottom of the list with about 1 million members and mainline churches have over 140 million. JW's have not had any success in growing any faster than mainline Christianity and they are definitely behind the Mormons in growth. Well, this is one statistic you won't see in the WT Yearbook.
Top 25 churches ranked by membership:
1. The Roman Catholic Church; 67,117,06 members; down 0.59 percent
2. The Southern Baptist Convention; 16,266,920 members; down 0.24 percent
3. The United Methodist Church; 7,931,733 members; down 0.80 percent.
4. The Church of Jesus Christ of Latter-day Saints; 5,873,408 members; up 1.63 percent
5. The Church of God in Christ; 5,499,875 members; no change reported
6. National Baptist Convention, U.S.A., Inc.; 5,000,000 members; no change reported
7. Evangelical Lutheran Church in America; 4,709,956 members; down 1.35 percent
8. National Baptist Convention of America, Inc.; 3,500,000 members; no change reported
9. Presbyterian Church (USA); 2,941,412 members; down 2.79 percent
10. Assemblies of God; 2,863,265 members; up 0.96 percent
11. African Methodist Episcopal Church; 2,500,000 members; no change reported
12. National Missionary Baptist Convention of America; 2,500,000 members; no change reported
13. Progressive National Baptist Convention, Inc.; 2,500,000 members; no change reported
14. The Lutheran Church – Missouri Synod (LCMS); 2,383,084 members; down 1.44 percent
15. The Episcopal Church; 2,116,749 members; down 1.76 percent
16. Churches of Christ; 1,639,495 members; no change reported
17. Greek Orthodox Archdiocese of America; 1,500,000 members; no change reported
18. Pentecostal Assemblies of the World, Inc.; 1,500,000 members; no change reported
19. The African Methodist Episcopal Zion Church; 1,400,000 members; down 3.01 percent
20. American Baptist Churches in the USA, 1,358,351, down 0.94 percent
21. Baptist Bible Fellowship International; 1,200,000; no change reported
22. United Church of Christ; 1,145,281 members; down 6.01 percent
23. Jehovah's Witnesses; 1,092,169 members; up 2.12 percent
24. Christian Churches and Churches of Christ; 1,071,616 members; no change reported
25. Church of God (Cleveland, Tenn.); 1,053,642 members; up 2.04 percent