Facebook seem to be testing a locked navigation bar so that users can scroll further into lifestreams. You still have to click to 'see older posts' but it certainly makes the site faster to use.
Locked Navigation Bar
Are you seeing this feature? In the screenshot below, you can see how the page is scrolling under a locked bar as half of the 'news' item is missing.
Another feature that we spotted is a mini notifications stream that appears in the top left corner. The unit is approximately three ad spots tall, updates in real-time, and infinitely scrolls on mouseover through all notications.
Unfortunately, we do not have a screenshot to show you that particular feature as it seems to have disappeared. At the time of spotting it, it was a little out of place, almost looking broken.
New Stories from 'Verbing' Buttons
One other interesting characteristic of this real-time notifications feature is that new types of Facebook 'stories' were appearing, which made new uses of standard actions within the Ui. These were in line with Facebook's tendency of 'verbing the web' - for instance, the new stories created sentences out of the 'attending' event feature, where you can click 'I'm attending' - the end result is a story such as "Michael is now friends with Lisa after attending the Summer Gala event".
Response to Google+
I think it would be premature to say that this is in direct response to Google+ but it is worth noting that the black "Google+ Bar" has been getting a fair amount positive and negative attention. On the negative side, users have felt that the +Bar has been imposed upon them, whilst on the positive side some users have liked it so much that they have expressed the wish to lock the Google+ Bar in place.
Either way, Google's black toolbar does seem to make better ue of screen estate to showcase all the different products (e.g calendar, email, docs etc) you might use.
Meanwhile, Facebook's UI does not easily show off all of the comparable features it has, such as events, messages, groups and applications. Despite the fact that there is little material difference (in terms of features) between the competing social networks, Google+ is winning the attention battle with its cache of beta testers over its simpler design. Therefore, I would not be surprised if glimpses of UI tests such as what we're seeing now foreshadow another Facebook redesign that pushes all its core services into more prominence.