Pinterest quarterly revenue, forecast disappoint; shares plunge

(Reuters) - Pinterest Inc fell short of Wall Street estimates for quarterly revenue on lower-than-expected average revenue per user and its revised full-year sales forecast came marginally below expectations, sending its shares tumbling 20%.Thursday's after-market plunge is expected to wipe out a majority of the gains since the online scrapbook company's blockbuster initial public offering in April, leaving the stock just about 6% from its IPO price.Pinterest on average posted revenue of 90..