A new study found that fewer than one in five smartphone owners access check-in services from their mobile devices, Mobile Marketer Daily reported. An estimated 16.7 million U.S. mobile subscribers patronized location-based check-in services on their phones in March, accounting for 7.1 percent of the total mobile population nationwide, according to market researcher comScore. When identified by smartphones, 12.7 million users checked in on those devices, or 17.6 percent of the smartphone population. ComScore defines check-in service users as those using services such as Facebook Places, Foursquare and Gowalla, all heavily skewed toward ages 18-34.