A fog of skepticism still swirls around Foursquare: Is the check-in service actually, y'know, a thing? Do people actually want to broadcast their location to the Internet—and use the resulting data to discover new places to go in the places they live?

A new set of data visualizations from Foursquare amount to the company's most extensive argument yet that people do actually use its service. The number of users have doubled since last May from 20 million to 40 million, though Foursquare has yet to disclose how many of those users are active on a monthly basis.

Increasingly, those users are international. Despite its American origins in New York City, Foursquare has found users as far afield as Beijing and Istanbul.

The videos show just how much they use it. And its users seem very ordinary, which is a good thing indeed: By showing how Foursquare users document their morning ferry rides, office arrivals, and departures by train, it demonstrates that the service has reached some segment of the mainstream.

Foursquare has also matured to the point of actually attempting to make money, by placing ads within its mobile app after a user checks in to a location.

The visualizations, highlighting global metropolises from Tokyo and New York to London and beyond, call attention to a different problem Foursquare faces: While it may work in some cities, outside of urban hubs, it struggles. The very density of activity we see in, say, San Francisco fades even as we venture into nearby suburbs. In exurban and rural areas, Foursquare is a ghost town. Without check-ins, its recommendations have little data to act on, and little interest to advertisers.

It's an open question whether Foursquare will ever work in such areas. Then again, if there are only three restaurants in a town, you probably don't need much help choosing between them—and they have little interest in spending on advertising to getting you to try them out.

Here's a view of Foursquare's urban universe:

Tokyo

Foursquare check-ins show the pulse of Tokyo from Foursquare on Vimeo.

Istanbul

Foursquare check-ins show the pulse of Istanbul from Foursquare on Vimeo.

San Francisco

Foursquare check-ins show the pulse of San Francisco from Foursquare on Vimeo.

New York

Foursquare check-ins show the pulse of New York City from Foursquare on Vimeo.

London

Foursquare check-ins show the pulse of London from Foursquare on Vimeo.