I really like Flickr, I think being part of the community has improved my photography, which is why I keep using it. But there is a price to pay, even though I use a Creative Commons license on my images, 20 or 30 images (to my knowledge) have been used inappropriately and it is extremely difficult to find out when this happens and almost impossible to do anything about it.
Flickr themselves are appalling in this regard.
Two totally inexcusable things that they do, which IMHO promote copyright theft, display their ignorance and devalue CC Licencing :
Flickr remove all EXIF tags (contains the copyright info) from your images that go into the download page (this is totally outragrous!!!).
Flickr allow you to change the licensing you add to an image after a license is applied ........ this is disallowed by Creative Commons, their licenses are by definition irrevocable.
However, lots of people do the right thing ....... I love it when someone writes to me describing their project, asking to use one of my images, I seldom refuse ....... and I have a growing collection of books and pamphlets at home, the results of some of those projects .......
So Flickr, get your act together, please !!!!!