Twitter is growing up by doing grown-up company things like acquiring its direct competitors as it has done by buying the microblogging platform Posterous. If you've never heard of Posterous, don't feel bad. The small San Francisco-based startup launched in 2008 as an easy way for people to start blogging and share photos and communicate with friends. If that sounds like a familiar concept, it's very similar to both Twitter itself, as well as a lot of other services (Tumblr, Pinterest, Path, to name a few.) And while there's little known about what will change as the result of the acquisition (Posterous says "Posterous Spaces will remain up and running without disruption"), it does ratchet down some of the logic for why Twitter would want to buy one of those competing blogging platforms. Also unknown is how much the Posterous investors will get from the deal. No terms were disclosed. But a quick glance at the Quantcast traffic chart show that Posterous has not taken off the same way Twitter and Tumblr has -- and even recent social media darling Pinterest has shown some signs of catching up, at least in Quantcast's estimates. (The numbers for Tumblr and Twitter are both indirectly measured estimates.)

As for what Twitter plans to use Posterous for, it's a total mystery. The last time they acquired a big-ish start up, TweetDeck, was almost a year ago, and they haven't made any discernible changes to their product or TweetDeck's. However, the company continues to charge down the path of expansion that they started traverse around the time of the TweetDeck buy. Now, if they could figure out a way to make money