The latest rage in social apps are private networks built for couples, the latest of which Avocado, was launched last month by ex-Googlers. But are lovers eager to commit their lives to each other through an app, even if it’s dedicated to fostering their relationship? Korean start-up VCNC is showing that some people are ready for just these kinds of services. The company’s app Between has just hit 1 million downloads since launching in private beta in November. It appears to be the first of the dedicated couple apps to hit that milestone.

The free app, available on iOS and Android, allows users to send private messages to each other, build photo albums based on shared pictures, manage anniversaries, compose special messages and leave memos for each other. Half a million users are active on a monthly basis and they are exchanging 4.6 million messages a day, uploading more 350,000 pictures a day and spending 300 minutes a month in the app.

So far, 76 percent of the apps downloads have been in Between’s home market of South Korea and another 15 percent is spread out through Asia. But the app is also growing in the U.S., where it has about 5 percent of its users and faces a lot of competition. In addition to Avocado, there’s Pair and Duet, two other apps aimed couples that have sprung up in recent months.

Jaeuk Park, the CEO of VCNC told me the there’s a growing need for this type of service for couples because many are worried about privacy on other social networks. Privacy was the most important feature cited by Between users in a survey. And he said there’s also a desire for couples to express their feelings for each other without wondering who else is watching.

“Some couples keep things quiet because they don’t want to show off. This is perfect for them because they don’t have to filter anything, it’s all private,” said Park.

Path also went down a similar road, trying to promote a more private social network. But Park said that’s still too open for many couples, who just want a safe space to communicate and build a scrap book of their memories.

As for how to monetize a private network for two, Between is looking at offering a premium version with video messaging, back-ups of pictures and animations. It’s also looking at mobile coupons and the sales of physical goods such as photo books and albums.

I’ve been trying Between out a little bit as a replacement for iMessage on the iPhone. It serves very much like a chat service but it has the added benefit of preserving all your pictures in its own feed with dates and location added. As long as I’m diligent about using it, it really begins to provide a nice service, giving me a private channel with my wife. And it has good replay value because you can go back and easily see just the images you’ve shared.

I think services like Between, Path, Avocado and others can grow because there’s still a lot of couples who could take advantage of a more dedicated communications and social tool. And this could also be lucrative as well. I imagine there are plenty of advertisers who would love to reach out to mobile savvy couples, offering them coupons on dinners for two, gifts and flowers.

Related research and analysis from GigaOM Pro:
Subscriber content. Sign up for a free trial.

• The connected planet: Smartphones aren’t the only player
• Social 2013: The enterprise strikes back
• Podcast: Mobile winners and losers in 2012 and what to expect in 2013