By Michael Johnston, founder of ETF Database.So much for a slow December. The latest National Stock Exchange numbers are out, and December was one of the best months on record for the ETF industry. Total cash inflows totaled nearly $30 billion, as investors flocked to domestic equities in the final month of the year. Nearly every issuer saw a big jump in assets in December, led by State Street ( STT), iShares and PowerShares. Total ETF assets increased to $790.9 billion, an increase of more than 5% over November's totals.