Didn't the users explicitly agree to the deal when they made an account and clicked "I agree to the Terms of Use"? Sure there's a degree of "oh nobody reads those anyway" to it that society at large should be approaching more rigorously (maybe a common-contract type of system), but at the end of the day the users are choosing to interact with a service, upload their content to it, and explicitly WANTING their content to be shared (to their friends, on their feeds, etc).
The real question is whether the agreement that was made between them and their users during 2007-now covered uses like training AI, but most likely it was a very broad "we can use your data how we want" type of statement and that agreement wasn't fought (or fought hard enough?) back then.
The real question is whether the agreement that was made between them and their users during 2007-now covered uses like training AI, but most likely it was a very broad "we can use your data how we want" type of statement and that agreement wasn't fought (or fought hard enough?) back then.