One particularly strange move in the tech sector that landed only yesterday was Twitter’s (News - Alert) unexpected announcement that they’d bought a company called Posterous (News - Alert). And this development is actually unexpected for a variety of reasons.

Posterous, in case you’re not already familiar, is a microblogging site much in the same style as Tumblr or even Twitter itself. Posterous put up a blog post assuring its user base that Posterous Spaces, the individual sections of Posterous filled with tiny blogs, would “remain up and running without disruption”, and that any significant changes to the service would be preceded by lots of advance notice so that the user base could make its necessary changes.

Reaction to Posterous’ side of the announcement ranged from the convivial to the horrified, as users alternately congratulated the staff of Posterous on their clearly upward mobility and expressed alarm that their happy home was about to be engulfed by Twitter. Some even referenced Tweetdeck, an earlier disaster for users that was likewise taken up by Twitter.

As to why you should be surprised by such a measure, the reasons are several-fold. First, some might be surprised that Twitter, which doesn’t have any visible advertising or the like, actually has the money to buy a company. Second is an issue of motivation; why would Twitter buy Posterous? The possibilities there are extensive: Twitter may be looking to add services, giving it not only the ultra-small update capability but also in-house blogging capability, making it a one-stop shop for its social networking crowd.

Also, Posterous offers up a service called Public Spaces, which allows users to automatically retransmit their photos and videos and the like to other social networks all at once, without having to resize everything to fit the various networks, making this a potential social networking hub. And if Twitter controls a social networking hub as well as being a social network, it stands to see its traffic count climb nicely. This keeps investors, and potential future investors, favorably inclined toward Twitter.

While there are numerous possible explanations to cover why Twitter picked up Posterous, the exact explanation is one only known to Twitter itself. Still though, it’s a development that bears watching as just where Twitter will take its new property is, as yet, quite unclear.