Image source: Square.2016 hasn't been a great year for Square (NYSE: SQ) investors. Year-to-date, shares of the payments processor have fallen almost 24%, badly underperforming the broader market. Most of that loss has been fueled by the company's recent earnings report. Although Square's business is growing rapidly, doubts remain about the company's long-term potential.Investors have many reasons to be optimistic, but should also be prepared for Square shares to head even lower.