In late September Google purchased mobile social-networking start-up Zingku. Now it’s gone and bought Jaiku, a Finnish company that offers an “activity stream and presence-sharing service” similar to the more widely known Twitter. Two months. Two mobile social-networking start-ups. Google never explained its plans for Zingku. What’s it going to do with Jaiku? Who knows …

“We plan to use the ideas and technology behind Jaiku to make compelling and useful products,” product manager Tony Hsieh wrote in a post to Google’s corporate blog this afternoon. “Although we don’t have definite plans to announce at this time, we’re excited about helping drive the next round of developments in Web and mobile technology.”

Ah. Helping to drive them the way you drove Dodgeball? To that happy place where social-networking apps go to die? Kidding, of course.

Anyway … Why did Google choose Jaiku over Twitter, a similar company with far greater brand recognition? “The answer seems pretty obvious to me,” says Tim O’ Reilly. “Jaiku isn’t a ‘lifestreaming’ company per se. They are a mobile company in the business of creating smarter presence applications. Far from being a runner-up behind Twitter, they are a leader in a category most people haven’t fully grasped yet. Google is clearly thinking a lot about mobile, and so they do grasp it.”

Of course, there’s another answer to that question as well. Twitter founder Evan Williams, whose previous company, Blogger, was acquired by Google in February 2003, may not have wanted to see another of his creations doomed to irrelevance by the search giant.

Which is not to say Twitter won’t be acquired. As RedMonk analyst James Governor points out, the company is probably looking pretty good to Yahoo right now. “Google and Yahoo are in dueling acquisition mode, and Yahoo is almost certain to respond,” said Governor. “Especially since Twitter has begun to use a footer on SMS messages it sends out–which could of course be used as a microbillboard.”