The smartwatch cometh 21.54% of Fitbit's outstanding shares were sold short as of March 15. That's a fairly sizable figure for a company that's turning a profit and trading with a reasonable valuation. Fitbit's trailing price-to-earnings ratio is just above 19, largely in line with the broader market. The problem is that Fitbit derives virtually all of its revenue from the sale of its wearable fitness trackers, a market that could dry up, or even vanish outright, in the years ahead.