File of people holding mobile phones silhouetted against a backdrop projected with the Twitter logo in Warsaw

People holding mobile phones are silhouetted against a backdrop projected with the Twitter logo in this illustration picture taken in Warsaw in this September 27, 2013 file photo. Twitter Inc on April 29, 2014, reported 255 million monthly active users, up 5.8 percent from the previous quarter but not enough to satisfy investors increasingly concerned about its struggle to gain a mass following. REUTERS/Kacper Pempel/Files (POLAND – Tags: BUSINESS TELECOMS LOGO)