As an avid Foursquare user in Seattle, Brian Groth knows he isn’t experiencing all the benefits that someone might see in, say, New York or L.A.. But at this stage, it’s more about the proof of concept, and the long-term potential of check-in services.

“I’ve gotten a free popsicle at PCC in Fremont,” he says, laughing. “That was exciting for me. ‘Hey, look at that, it actually worked!’ … My wife just shakes her head.”

The key, of course, is the network effect. The more people and businesses take part, the more rewarding the experience for everyone. Which is why Groth and some of his fellow Foursquare aficionados are organizing Seattle’s installment of the global Foursquare Day tomorrow, April 16.

The day — chosen because 4 squared =16 — was inspired last year by a Florida optometrist. Meetups will be held around the world on Saturday for Foursquare users to connect with other fans and galvanize support for the future of check-in services.

Foursquare Day this year comes as some people question the ability of the check-in phenomenon to sustain itself. In a recent post on ReadWriteWeb, “2011: The Year the Check-In Died,” entrepreneur Mark Watkins analyzed the trends and concluded that “unless ‘normal’ people find direct, personal value in the service, they’re not going to adopt it and the service will remain as a toy for the tech-obsessed.”

Groth, who works in technology and marketing, sees the potential in emerging technologies such as Near-Field Communication, or NFC, which will allow smartphones to automatically check people in if they’ve authorized the capability in advance.

“As more and more people are out and about with their smartphones, that means marketers want to take advantage of that,” he says. “Services like Foursquare are a manual thing right now, but eventually you’re going to get to the point where you can have them check in for you, without you doing anything, and if there’s a special, pop it up on your phone.”

In the meantime, there’s always those free popsicles.

Seattle’s Foursquare Day event starts at 12:30 p.m. Saturday at Gordon Biersch at Pacific Place downtown. Specials will be available based on the number of check-ins, of course. Organizers are also planning a raffle, with prizes, and related downtown Seattle and Capitol Hill pub crawl, with drink specials for checking in.

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