Compare and contrast: Pair is a Y Combinator grad and raised $4.2 million in seed funding from a group that includes Dave Morin, Paul Buchheit, Yuri Milner, Gary Vaynerchuck, and Ashton Kutcher’s A-Grade Ventures. Based in Silicon Valley, it provides a private communications app for couples. The app launched in March and has been available for Android since May. By then, it had been downloaded about 220,000 times. It is available only in English.

Between was started by five friends in Seoul, South Korea. It launched as an iPhone app in November last year, with $1 million in seed funding from Softbank. It launched in four languages – Korean, Japanese, Chinese, and English – and now boasts 13 languages. It has been available on Android for months. Today, it hit 1 million users.

I met Jaeuk Park, one of Between’s founders, and two of his colleagues, Edward Lee and Erin Chang, for dinner last night in Seoul. They’re in their mid-twenties and are smart and animated, with Lee and Chang sporting American qualifications and accents. Inspired by Facebook COO Sheryl Sandberg’s clarion call to climb aboard a rocketship, Chang decided turned her back on Harvard Business School to join VCNC, Between’s holding company.

There are several key differences between Pair and Between. The most significant is that while Pair is an in-the-moment service, Between focuses on relationship history. For instance, it lets users build albums by selecting their favorite photos. VCNC has partnered with a photobooks company called Snaps in order to let couples create and buy physical albums filled with their photos.

While Pair offers live-sketching, short videos, and the playful “thumbkiss,” Between has taken a more stripped-back approach, focusing on photos, anniversaries, and letting users share longer notes with each other.

It has, however, been quick to launch a key monetization feature, an events box that serves the dual purpose of being an advertising channel and an ecommerce portal. Through that tab, businesses can offer discounts on products and services. Samsung’s Everland theme park is among the first partners to use the feature, which has been available for the last couple of months. The company also plans to add premium options, such as long videos, high-resolution photo back-ups, and animated emoticons.

Rather than be put off by the launch of Pair, which arrived four months after Between, the VCNC team saw it as a surprising validation of their idea. Until that point, they were unsure how well an app like Between would work in the US, where couples’ dynamics are different than in Korea. In the US, they figured, couples aren’t so shy about sharing their relationship on wider social networks, such as Facebook and Twitter, and so perhaps wouldn’t see as great a need for a private communications platform.

“It was a really good signal for us, because many people thought that Between was just an Asian thing,” Park told me. “But we saw that there’s a market in the US too.”

The US has been a prime target for VCNC since day one, but it is not going to rush into the market without being prepared. It is perfecting its product first in Korea, where 76 percent of its users are based. Today, China (7 percent) and Japan (5 percent) are its next biggest markets. “We know the Asia market better than the US market,” Park said. “We want to dominate the Asia market first.”

So far, it hasn’t spent a dime in marketing to the US, but that will change in late 2012, when it will open a San Francisco office and kickstart its marketing for an American audience. When it gets there, it will have more than just Pair to worry about. Private social networking is becoming hot, with Cupple, Every.Me, Just.Me, and Avocado all leaping into the space.

Two may be company, but Between will have to be ready for a crowd.