If you can’t join them, beat them.

That seems to be the logic behind Facebook’s new set of APIs.

After its rumored $500 million offer to buy out Twitter was rejected, Facebook instead released changes to its platform, to make it easier for third-party software to access Facebook “posted items” — so that status updates, notes, photos and videos can now be shared across other platforms.

We’ve seen increasing engagement with over 15 million users updating their status each day and sharing over 24 million links per month. We wanted to make sure this content and the ability to share this content was available through our standard APIs… In addition, we’re opening new APIs for you to post links, create notes, or upload videos for the current user, and we’ve made setting a user’s status easier.”

Facebook’s posted items are, of course, a form of microblogging — and the company’s interest in publishing this content more widely is seen as a shot across the bow at Twitter.

(For perspective — and despite all of the hype that Twitter’s received this year, and the enormous tenfold growth of its user base — Facebook boasts twenty times more users and twenty-five times as much funding.)

Meanwhile, microblogging is all a-twitter on Facebook lately, thanks to the spread of a viral chain letter, “25 Random Things About Me.”

Rules: Once you’ve been tagged, you are supposed to write a note with 25 random things, facts, habits, or goals about you. At the end, choose 25 people to be tagged. You have to tag the person who tagged you. If I tagged you, it’s because I want to know more about you.

During the last week, over five million of these lists have been written and posted to Facebook: five million people — many of whom would not otherwise consider themselves to be “bloggers” — typing out their thoughts, sometimes personal, sometimes sublime, and then publishing them  to their community.

A big week for microblogging, and for Facebook.