Youtube is a popular site to view unreliable, low-quality streaming versions of unlawfully shared videos. Nobody thinks Youtube is a substitute for (say) a Hulu+ or Amazon Prime subscription.
Maybe not, but for many people it is a replacement for MP3 downloading services like iTunes. Sure, it doesn't offer all the same features (neither did MU), but it offers enough.
People also shared legitimate content on Napster. Once in a blue moon, someone will use BitTorrent to transfer a Linux kernel. But we all know why those services were or are popular.
In my (anecdotal, yes) experience of watching people use it, that's exactly why Youtube is popular too. User created content is an exception.
There's a distinction to be drawn between copyrighted content and copyrighted content of such quality that serves as a market substitute for the legitimate product.
In either case, the DoJ has huge discretion about what cases it brings. A major component of that discretion is its belief that it can win the case (much harder against Youtube); another component is "what offender is so weak that we're likely to maximize the precedential value of the case"; another is "how much of a message will this send".
Maybe not, but for many people it is a replacement for MP3 downloading services like iTunes. Sure, it doesn't offer all the same features (neither did MU), but it offers enough.
People also shared legitimate content on Napster. Once in a blue moon, someone will use BitTorrent to transfer a Linux kernel. But we all know why those services were or are popular.
In my (anecdotal, yes) experience of watching people use it, that's exactly why Youtube is popular too. User created content is an exception.