The ETF juggernaut rolled on in December, crushing mutual funds on its way to an 11th straight month of net inflow. Investors plowed a whopping $50.8 billion into the nation’s equity, bond and hybrid exchange traded funds combined. That pushed U.S. ETF industry assets to $1.974 trillion at the end of December, up 1% over the previous month, according to Investment Company Institute, a trade association.