On the conference call this morning discussing DST’s $200 million preferred equity investment in Facebook, CEO Mark Zuckerberg mentioned that while advertising is currently Facebook’s largest revenue stream by far, the company is looking at microtransactions as a potentially significant future business.

“We’ve tested and are continuing to test a number of things with user payments,” Zuckerberg said. “It’s not a large part of the business now, but we hope it can be in the future. It’s interesting that around the world, these products are creating a lot of value for users, and there’s a lot of different ways to monetize them.”

Facebook’s payment initiatives have been speculated on for a long time. In late 2007, Facebook announced that it was beta testing a payment platform for developers. However, those plans have since apparently been tabled, and Facebook instead appears to be moving in a “universal virtual currency” direction. Under this approach, users would pay Facebook for virtual currency (called Facebook “credits”), which they can spend in the Facebook gift store or in any Facebook applications which have integrated Facebook credits. Facebook confirmed to us two weeks ago that it will start running virtual currency integration tests with application developers soon.

As an indication of Facebook’s current plans to build out its payments initiatives, the company is now looking to hire a “Manager, Payment Systems” that will have “broad responsibility for both expanding our business metrics such as accessibility of payment marks, revenue enablement via payments, and processing costs (including interchange and vendors).”

The recently posted job listing says this hire will “serve as cross-functional company lead for payments initiatives, including driving strategy for payments acceptance,” and will be responsible for “expanding available payment methods to drive incremental revenue.” In addition, this hire will “structure, negotiate, and manage relationship with payment partners and vendors” and should have “knowledge of stored value payments systems and instruments such as gift cards, virtual currencies, or similar.”

As Facebook increasingly becomes a global business, user micropayments offer a promising way to build revenues in countries with significantly lower per-capita GDPs than the US and other markets where advertising can drive big revenue numbers. New Facebook investor Yuri Milner has portfolio companies with a lot of experience monetizing social networks in Russia and eastern Europe through various forms of user payment, so Facebook may be looking to Milner for advice.

Now, Facebook has the opportunitiy, along with competitors like DST portfolio company VKontakte and global social network hi5 (which also recently launched a universal virtual currency for its application platform), to figure out how to make e-commerce work at low price points across highly fragmented markets. Developers are welcoming Facebook’s work in the space.

“We think that solving [the payments] problem is where the platform has unique advantages of scale,” Playfish COO Sebastien de Halleux recently told our sister site, Inside Social Games. “I’m not sure you can beat the platforms in terms of payment friction.”

As always, we’ll keep tracking Facebook’s monetization plans closely. The full job description is below: