Foursquare ad Dennis Crowley Naveen SelvaduraiDan Frommer, Business Insider

The latest version of Apple's mobile operating system, iOS 5, is a boon to Foursquare because it will drive up usage heavily. 

How do we know this? 

First, let's backtrack and look at what made Foursquare so successful. 

The idea of some sort of mobile/location-based social network has been around for as long as social services and cell phones. 

The idea didn't take off until Foursquare, though, for two reasons. One was because there wasn't an open, rich mobile operating system like iOS and Android.

But the other is that people envisioned services like that as broadcasting your location to your friends all the time, which Silicon Valley geeks think is awesome but the rest of the world thinks is intensely creepy. 

Enter Foursquare, which popularized the concept of the check-in: voluntarily announce your location to your friends! And we'll add game mechanics, social features, and eventually services like city guides and loyalty cards, to make it worth your while! 

In the way that only certain products can, Foursquare and the check-in "clicked" with people in a way no mobile social service had before it. 

But the "opt-in" nature of the check-in is both a strength and a weakness. Because you have to actively check-in to places, that's a barrier to engagement. Plenty of people are talking about "check-in fatigue": people sign up for Foursquare and start checking in because it's new and they're curious, but after a while they sort of give up.

Enter iOS's new notifications. Instead of a big, screen-consuming alert whenever you get a notification, there will be a small alert at the top of your phone. You can slide down with your finger to open a "Notification Center" where you can see all your notifications. It's much less annoying and a much better user experience.

Why is that a big deal for Foursquare?

The best way of dealing with "check-in fatigue" is to keep Foursquare "top of mind" for users when they're out and about. That's the goal of the "Check-in with Foursquare" stickers the company mails out to merchants for free all the time.

Notifications will do that in a big way, and this story explains how: last month, your writer went to have a beer with someone. I was early so, with nothing else to do, I checked in. A friend of mine saw the check-in and commented on my check-in inside the Foursquare app, saying they were really close-by and would I like to have coffee after I'm done--exactly the kind of serendipitous interactions that Foursquare is meant to enable. 

The only trouble was I only saw my friend's comment after I was back home, because I had turned off Foursquare's notifications. I may like Foursquare and have good friends on there, there's actually only a small amount of time when I want to know where they are. So I turned off Foursquare notifications in the iOS settings to not get pinged every 5 minutes when a friend checks in to the supermarket. And I suspect I'm not the only one. 

The point is this: the less intrusive mobile notifications are, the better it is for Foursquare, because people will be more inclined to keep them turned on. They might not interact with most of the notifications, but they will see them a lot more, which will keep Foursquare "top of mind" for them. Which, in turn, will ensure that they use the service more and have more "aha!" Foursquare moments like the one your writer missed because of turned-off notifications. 

This is actually a pretty big deal for them.

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