Many organisations collect and have lots of data about me.
The difference with Google, FaceBook, etc. is that they have far more data from far more different sources and they have far more money and far more power to put all together (this is the real danger) and get a picture of me, even I do not get.
And now they use this to increase their power and income and they do use this to manipulate elections, the thoughts, feelings and actions of people way more then ever in history before.
Anyone who does not fear this is willingly blind.
I am not religious, but if I was, trusting G, FB, etc, is like trusting the devil itself. Making a deal with him and selling their fortune to him.