Aeria Games announced today that it has raised an undisclosed amount of funding from Sony’s So-net Entertainment subsidiary. The Santa Clara, Calif.-based maker of multiplayer online games will use the money to expand further in the fast-growing free-to-play segment.

Aeria has quietly grown to more than 40 million users, and it continues to add about one million new players a month. It now has more than 300 employees and more than 30 free-to-play titles.

So-net, which is an entertainment-focused Internet service provider with more than 4 million subscribers, is now the second external investor in Aeria (in addition to NHN which invested in 2011). So-net is the largest shareholder of DeNA and the owner of Japanese game publisher Gamepot.

In March, Aeria announced that it planned to expand in the mobile game market with the establishment of its Aeria Mobile division. It has already launched four mobile games.

The company was founded in 2006, and it now has a presence in free-to-play online games in North America, Latin America, and Europe. In contrast to Zynga, which focuses on casual Facebook gamers, Aeria goes after hardcore players who enjoy intricate games in the action, role-playing, shooter, real-time strategy, and racing genres.

“We welcome So-net Entertainment Corporation as a strategic partner as we move forward to expand our relationships with global digital entertainment companies,” said Lan Hoang, Chief Executive Officer, Aeria Games. “Our track record and accelerated growth has enabled us to align with companies that will complement our portfolio, so we look forward to working with So-net and its partners to bring more highly sought-after content to our more than 40 million dedicated users.”

Aeria is expanding in regions such as Latin America, Eastern Europe, and Russia. The company has major development studios in Berlin and Sao Paulo, Brazil. Rivals include Nexon, GameForge, and Bigpoint.

The company was created by Hoang and Terry Ngo, two former Stanford University students. Hoang worked in Japan and often visited Ngo in the Bay Area. Both were active during the Internet boom and saw how the web was transforming many businesses. They felt hardcore game companies were ignoring Internet opportunities. In response, they started Aeria games using the free-to-play business model where users play for free and pay real money for virtual goods.