Social gaming giant Zynga has acquired Serious Business for an undisclosed amount, the companies just announced this morning. Both companies are based on San Francisco, and the Serious Business team has already joined Zynga at its Potrero Hill headquarters.

Serious Business has a very talented team and has built some hits, especially Friends for Sale, but it’s not huge. It has a total of 6.20 million monthly active users and 838,000 daily active users, according to AppData. The company previously raised $4 million from Lightspeed Venture Partners, which is also an investor in Playdom and RockYou. Serious Business’s founders, Alex Le and Siqi Chen, are both former employees of Powerset, a search company that Microsoft acquired in 2008.

Zynga currently has more than 68 million non-deduped daily active users on Facebook, according to AppData, making it by far the largest social game developer on the platform. We estimate the company brought in more than $200 million in revenue last year; it also recently raised $180 million from Russian investor DST.

Serious Business is not the first company that Zynga has acquired. Others include MyMiniLife, a smaller studio that has helped Zynga build popular simulation games like FarmVille, the Facebook app YoVille, and GoPets, a pet-caring game builder that appears to have helped work on the company’s PetVille game. The previous acquisitions were notable for their talent and strategic value to the Zynga product pipeline and that appears to be the case today.

Zynga’s other purchases have also led to new games, and we’re interested to see what results from the Serious Business team. Zynga filed 21 new game trademarks in December, so we’re expecting more, soon.

As social gaming becomes more competitive, we expect to see more smaller developers get acquired — either by larger developers like Zynga, or by other gaming companies.