Are they really Christian, or the usual syncretism?
I think this question (though I don't know what syncretism is) is a good way to go about discerning the issues. Falling away and growth of the church are dynamic principles. It's hard to say, "this is happening" or "this isn't happening" because they are both happening off and on constantly. People get deceived and fall away all the time, every day. People make choices to become sincere and try harder to follow the principles of Jesus every day, thus a "falling away" and a "growth" are both happening at the same time.
It's like that in the Revelation, too. Paul talks about how the end will not come without a great falling away, while at the same time we hear about this 144k faithful believers being hammered together into God's end time church along with a "great multitude no one can count", while Jesus also said the way is narrow and there are few who find/follow it. The only way to consistently arrive at the truth is to remain flexible in how we interpret the various teachings and events around us.
Anyone who relies on, "the Bible says so" will almost certainly end up fooling themselves into thinking that their little piece of the puzzle equates to the bigger picture.
As for "The Great" falling away, I think it will probably have a lot to do with insincerity and luke warmness. Jesus talked about people rocking up at the gates wanting to be let in, but being denied. They obviously believed they should be let in, meaning that these are people who thought they were right with God. He asked the people, "why do you call me Lord, but no not obey me".
He talked about a time (i.e. the end time) when people would kill us, "thinking they are doing God a service". He talked about the sheep and the goats, with the goats being people who believed they were right with God, but being rejected. He quoted Isaiah who said, "This people draw near to me with their lips, but their heart is far from me".
The False prophet is described as having the "horns of a lamb" but "speaking with the mouth of a dragon", suggesting that the False prophet will probably claim to have the authority of Jesus, but will instead teach contrary to the teachings of Jesus.
I believe the Great Falling away will be a worldwide movement toward false peace and sincerity which is disguised as genuine faith. In the Revelation Jesus talks about luke-warm people, and how much God hates the luke-warmness. Imagine people going around claiming to represent you, but teaching and behaving contrary to what you actually believe and teach.
I've encountered hundreds of people who are sick of Christianity, and when I ask them why, they cite the prosperity gospel, hatred for homosexuals, self righteousness, false healings/miracles, bigotry, and any number of other genuine complaints, but the most frustrating thing is that they express the complaints as though this is what Christianity really is all about. In many cases this kind of reasoning becomes a convenient doctrine to accuse Christianity as a result of their own personal biases, but there's plenty of examples where the testimony of professing Christians who act contrary to what Jesus taught really do turn people away from God.
The "falling away" will certainly include professing Christians, but I think the damage will be much greater concerning those looking on who might have been inspired by genuine faith, but only see greed, hypocrisy and self righteousness being justified by "faith".