Nearly one in five smartphone users are tapping into check-in services like Foursquare and Gowalla, according to a ComScore report released yesterday.

A total of 16.7 million mobile-phone subscribers used location-based services on their phones in March. That amounts to about 7.1 percent of the entire population of mobile users.

But among smartphone users specifically, 12.7 million checked in with such sites during the month, representing 17.6 percent of all smartphone owners, ComScore said.

For the purposes of its study, ComScore included such location-based check-in services as Foursquare, Gowalla, and Facebook Places.

Compared with most smartphone owners, those who used location-based sites on their phones also proved more likely to access other types of mobile content. More than 95 percent of them used their mobile browsers or mobile apps. Almost 62 percent grabbed the news on their mobile devices.

And in a stat that will make retailers happy, almost one third of the users browsed to online retail sites on their phones, while one fourth accessed online shopping guides. Users of check-in sites also got a heavier dose of advertising. Almost 40 percent of respondents said they remembered seeing an online ad during March, compared with just 27.5 percent of all smartphone users who said the same thing.

"Although still in their relative infancy, location-based mobile check-in services are seeing rather impressive adoption among smartphone users," Mark Donovan, ComScore senior vice president of mobile, said in a statement. "The ability to interact with consumers on this micro-local level through special offers, deals, and other incentives provides brands with the real-time opportunity to engage consumers through their mobile device."

ComScore based its results on data from its MobiLens service.