All this show is that more people do not ally themselves with a particular denomination. It does not take into account the growing number of non-church attending Christians. It does not mean that Christianity is on the wane but that many established churches are.
You've hit the nail on the head. As the clerical leadership of the old line Protestant churches have drifted further and further to the left and have become fixated on secular matters so they have shed more and more of their congregations. The Episcopal Church ( Full disclosure I'm a member) is rapidly declining into irrelevance. Divorce is OK. Homosexual marriage is OK. There is no longer a moral backbone provided by the church for it's regular members. I'm coming to the belief that it's better to belong to one of the evangelical churches except I haven't figured out which one yet.