When it seemed like things couldn't get any worse, the stock collapsed.After a sharp rebound over the summer (from lows made during the tariff-driven sell-off in April) Pinterest's (PINS +2.06%) latest earnings report knocked the stock lower again. What spooked investors? Management issued worse-than-expected revenue guidance for the important holiday quarter.The stock's massive drawdown on Wednesday brings Pinterest's long, disappointing five-year return back into focus. Is the stock simply not investable? ...