Found via Ten Reasons -

This report of the National Council of Churches shows the 25 largest Christian denominations in the US. The Catholic Church is the largest Christian Church and grew by .87 percent. The report also states:

"Jehovah's Witnesses and the Church of Jesus Christ of Latter-day Saints, regarded by many Christians as cults, reported the largest membership increases in a year, according to the National Council of Churches' 2008".

The Mormons (Latter Day Saints) have large families so that would probably explain their growth. The Jehovah's Witnesses however simply do the evangelical work that Christians are called to do. It is easy to make fun of them for their door to door work, (see Barats & Bereta video The Good Word) but apparently it does work. Around here they are very successful with evangelizing minorities. At the train station my wife and I sometimes use there is usually a nice, elderly gentleman handing out Watchtowers. It is too bad their theology is so unorthodox. The Catholic Church has a long way to go with evangelizing, and in most cases, "re-evangelizing". Meanwhile, growth is not happening in some Christian groups:

"The greatest losses in membership were reported by The Episcopal Church, which dropped 4.15 percent in members, and the Presbyterian Church (U.S.A.), which decreased by 2.36 percent. Both denominations are currently wracked by theological differences and the issue of homosexuality."