Investing.com - Exchange traded funds are more popular than ever with investors, and millennials are particularly fond of them.That's the conclusion of Charles Schwab (NYSE:SCHW)'s eighth annual survey of ETF investors.The survey found that more millennials say ETFs are their investment vehicle of choice than investors in general. Millennials also have a higher percentage of their portfolios invested in ETFs.In fact, ETFs are so popular among millennials that a majority of them have already replaced all of the individual stocks in their portfolios with ETFs. ETFs hold baskets of securities like mutual funds but trade during the day like individual stocks. They've become popular with investors because their passive management structure allows them to charge lower fees. Global ETF assets rose by 35% in 2017 to $4.569 trillion.