How many people on the internet post fake pictures of themselves?

I have noticed one thing on the internet. There are people who steal pics and post them as themselves. People who steal pics of models and put them on their profiles and say it is a pic of them. How many do this? It is easy to find out because you can find the same pics on sites like flickr and they are definitely from a different person. Why do they do this? Do they not know that people will eventually find out?