I stopped posting my photos to facebook about a year ago. There, your friends probably don’t understand that your photos are your artwork. I had a friend who told me after her presentation that my pictures helped her Spanish project. I would have had no problem with it if she had asked to use my work. I probably overreacted, but I felt as if she had stolen my work. And I never would have known, had she not told me.
On Flickr, most members of the community are also posting their artwork and understand creative commons and proper etiquette that comes with using images posted by others.
If I want my facebook friends to see the pictures I post, I’ll update my status with a link to the newest flickr updates.