They brought the heat on themselves when they decided to mock DMCA requests and cease and desists instead of accommodating them.
There's a reason 4chan, Reddit, imgur, Tumblr and its ilk all still exist in the age of copyright. None of them produce their own content-- it's all user submitted, and mostly in violation of some copyright or another.
Generally speaking, my understanding is that hosts aren't liable for user-uploaded content unless they curate or promote it (deletion notwithstanding). That changes their role to that of a content distributor/publisher instead of a mere platform. This is why backpage's CEO got arrested for human trafficking whereas craigslist's did not-- when challenged craigslist shut down its prostitution ads, but Backpage actively reworded and posted them and in doing so became their publisher.
