Retweeting is one of the several phenomena that have just evolved on Twitter simply as more and more people have started adopting it. Like many of the best features on Twitter, you could argue even the service’s purposes, as it wasn't planned on, it was a good tool so people used it. A couple of months ago, Twitter decided to make it a permanent feature, so it devised a slightly altered retweet system that it integrated with the site and the API. Unfortunately, it hasn't had much of a reception and the feature hasn't been too popular so far. This isn't stopping Facebook from, once again, 'borrowing' from the fledgling 'information network' by introducing its own take on the retweet functionality.
Facebook has started testing a very similar feature, which enables users to share stories from their friends, but also credit them for the stories with a "via" link under the post. The feature is apparently in testing, though it looks like it's live for everyone. Users can now repost a news feed entry they found interesting from their friends on their own feed.
When clicking on the "Share" button, users will see the "via [friend name]" line in the sharing dialog. They can choose to remove it, but, if they don't, a line will show up below the entry indicating its original author. The feature is limited for now, it only works for posted links, one doesn't get the 'via' option for regular status updates or photos.
Facebook hasn't provided any official details so far. It may be that this is just a first step and that the social network will roll out the feature for all news feed items and status updates, though it probably is best suited for the types of posts it is currently enabled for. Whatever the case, it's clear that Facebook's 'Twitterification' is not done, though there aren't that many features left on Twitter from which the social network hasn't been 'inspired.'