What percentage of airliner flights globally are freight flights?
The National Air Traffic Controllers Administration states that:


  "On any given day, more than 87,000 flights are in the skies in the United States. Only one-third are commercial carriers, like American, United or Southwest. On an average day, air traffic controllers handle 28,537 commercial flights (major and regional airlines), 27,178 general aviation flights (private planes), 24,548 air taxi flights (planes for hire), 5,260 military flights and 2,148 air cargo flights (Federal Express, UPS, etc.). At any given moment, roughly 5,000 planes are in the skies above the United States. In one year, controllers handle an average of 64 million takeoffs and landings."


Therefore, if there are 28,537 commercial flights and 2,148 cargo flights;


28,537 + 2148 = 30685
(100 / 30685) * 2148 = 7.00016294606%


Thus there's for all of the airline flights taking off daily, 7% of them are cargo, and the rest are commercial.