Expectations are low ahead of Twitter's (NYSE: TWTR) first-quarter earnings release this week. Shares are down about 10% year to date and are trading at under $15 at the time of this writing -- not far from the stock's 52-week low of $13.73 and light years away from levels above $50 seen as recently as two years ago.An earnings preview of three important areas of Twitter's business to examine when the company's first-quarter results go live reveals some insight into the market's bleak expectations for the stock.