You can monetize content as long as you make good faith attempts to remove it when you get alerted about it according to the DMCA.
Because they're facebook. They have no competition.
It's doomed to fail. YouTube allows creators to profit and build businesses for themselves. Facebook shares none of it's revenue with creators and this is not a sustainable model in the long term.
They will be sharing the 6 second proceeds with creators to encourage more creators to post content to Facebook.
Even if they are sharing with uploaders, they're less creators and more freeloaders pirating other people's YouTube content in most cases so this is pretty irrelevant at best.
Because they haven't done enough to piss off the RIAA/MPAA and get a billion dollar lawsuit yet. They're only abusing smaller creators who can't afford to do anything serious about it.
Good point. And Facebook arguably has more money to throw at their lawyers than anyone else (apart from Apple and Google)