Facebook warns it is 'underreporting' iOS ad results amid Apple privacy changes

FILE PHOTO: A 3D plastic representation of the Facebook logo is seen in this illustration in Zenica, Bosnia and Herzegovina, May 13, 2015. REUTERS/Dado Ruvic

(Reuters) - Facebook Inc said on Wednesday it is underreporting the results of its advertising business on Apple Inc iOS devices, acknowledging that recent privacy changes from the iPhone maker have made it more expensive and difficult for brands to advertise on Facebook.

Shares of the social media giant were down 3.7% at $344.32 in morning trading.