Sure they can. Stack overflow is one example. Any business operating on user driven content will be culpable. When you agree to the EULA and it tells you "what you post here belongs to us and we grant you a license to publish it yourself", you're signing over ownership of your content in exchange for a license to replicate it. That's how social media all works, all the EULAs work that way. FOSS is no different.
bananaghost