AOL has announced the purchase of Third Screen Media, a private company that specializes in placing mobile advertisements on cell phones, a move suggested by some to be in direct response to Microsoft Corp.'s purchase of a similar technology company a couple of weeks ago. The two latest moves indicate that the mobile market represents significant potential for wireless carriers and advertisers. According to figures from eMarketer, US annual spending on mobile advertising will skyrocket from US$903 million to more than US$4.7 billion over the next four years. Global spending is expected to near US$14 billion at the end of the same period.