I always felt with much of my personal work on the 'net, and never minding, that if I put it there, I can't mind if people use it and do not credit me. I also felt that if something I want to use is the first or near the top of a Google search, that either others are probably sharing this as well, or the poster knows about this and/or doesn't mind.
However, I have been scolded often about posting lyrics, and felt guilty, and started crediting all song lyrics. Later, most or all of the musicians joined my friend's list on MySpace, and some actually befriended me to some extent. Some were always connected to me. When I explained this online recently, I got more and more grief and realized this was to be.
Now, I want to always credit my source of every image I clip and use, but feel I need to think long and hard about whether to pursue the people who clip and use my art.
My other option of removing most everything or leaving the 'net is too extreme at this point. I never defriend people either, but recently did with two here, as one was harrassing ladies I knew and the other changed his or her screen name to (an evil reference). In the past, I was always cool with that and almost anything, but began to get much grief regarding connections to others of that nature.