Although the Japanese tech giant Softbank is now best known in Silicon Valley for its massive $150 million investment in Zynga, the company is also active in smaller rounds. MocoSpace, a growing mobile social network, is the latest to get one.

MocoSpace is announcing a $3.5 million addition to its second round of funding from SoftBank today. The aim is to use the money toward a new initiative: browser-based gaming on mobile devices.

That sounds like an odd idea — mobile gaming has been strongly tied to downloaded apps since the advent of smartphones. But MocoSpace sees an opening. “We think there’s a huge opportunity, and it’s an uncrowded space,” says CEO Justin Siegel.

There may be something to the idea for MocoSpace. Most of its six million monthly uniques visit in a browser, and Siegel says there’s “strong user interest” in gaming.

The question of what kind of social games mobile users would play in a browser window is also important. Siegel thinks that Facebook-style social titles may well be successful. “Users are logging in first for the social, then gameplay is a close second,” he says.

Aside from MocoSpace’s relatively small size, there will be one major challenge for developers looking to put a game on the platform: the technology. Developers will have to use WebKit and HTML5, which many may find unfamiliar.

However, some developers are interested in learning to use HTML5 anyway, while there are also service firms created to port into HTML5, like Moblyng. “The smaller development shops we’re talking to are saying, A it’s an opportunity to make more money, B leverage existing assets, or learn more about mobile,” says Siegel.

Including the SoftBank investment, MocoSpace has raised $10.5 million to date. The company was founded in 2005.