PopCap is a venerable casual gaming company. It bootstrapped itself up over the past decade without ever raising a round and it has an unbroken string of hits — titles like Bejeweled, Zuma and Plants vs. Zombies. And now, it’s making a big move into social gaming, raising $22.5 million from Meritech Capital Partners, along with investors Larry Bowman and John McCaw.

The Seattle company has recently been focusing on Facebook, and relatively successfully. It launched Bejeweled Blitz on Facebook last Christmas, based on the original puzzle game. The app grew fast over the spring and summer to plateau at around 7.41 million monthly active users today. It launched Zuma, a pinball shooting game, on Facebook in July, and also built a version on its own site using Facebook Connect. However, it appears to have taken the app offline in mid-August to focus on the Connect version.

The company pioneered a business model where users download a game and play it for an hour, but they have to pay to unlock it if they wanted to play longer, as Dean Takahashi notes over on VentureBeat. The company has sold more than 25 million units of Bejeweled alone this way. But social games are free, and make money on virtual goods.

This is why the 260-person, well-established company decided to raise another round. Here’s the statement from chief executive David Roberts:

We’re excited to have additional working capital that lets us be more aggressive with our expansion into social media and reaching new geographies. Meritech is a premier late-stage investment firm with a portfolio that includes some of our most important business partners such as Facebook, and adding John McCaw to our investor roster is of huge strategic importance with his deep connections in the mobile and financial communities. We’ve been pursued by investment firms for many years and have resisted taking outside capital, but we liked Meritech’s style and believe there’s a tremendous opportunity to grow and evolve our business at a time when many other video game firms are retrenching.