Copyright infringement in Social Media?

I was planning to promote my site through Tumblr and Facebook. As you may know, image posts are highly regarded in these sites. And I'm pretty sure few of them are actually "Free to use". So what I'm wondering if there have been any known cases where Facebook/Tumblr users have been sued over copyright infringement? In personal uses of social media I wouldn't care, but since I'm trying to stay legal with my main site and my actions on Tumblr/Facebok will link back to it I'm wondering if it will pose a risk. I do know that it is illegal in theory, but considering the high volume of users doing this my question is rather: Should I care? Do you guys use social media and do you worry about copyright infringement? And yes, once again: any known cases where other people have been in trouble?