Facebook is adding offline viewing for some videos, with a new option in the settings of the app to cache certain videos so they can be watched later even if the user doesn't have an internet connection. The new option can be seen just under the toggle for uploading videos in HD, and is labeled as an ability to watch some videos without using mobile data. If you frequently or at least from time to time end up without a connection to your cellular network, then this is an option you're going to want to toggle on, that is if you watch videos on Facebook in the first place.
In addition to the new caching option for videos, Facebook is also going through with changing the look of a few of the icons used to interact with posts. For liking, commenting on, and sharing a post there will be a new set of icons soon, although these aren't terribly different from the current icons that most users are still probably seeing. The like button is more or less the same shape as is the share button, but the comment button is now a rounded chat bubble instead of the button with a more squared off shape that simply has rounded corners. The inside of the icons are also white, and it seems that Facebook has used a different font for the words for those icons, and the color of the icons as well as the font seem to be darker than before as well.
Visually these are just minor changes and they don't really affect the way that the app will function, as liking a post will still like a post, and hitting the comment or share buttons will still allow you to comment on or share a post. These appear to be the only new changes to the app at this point but since all users aren't seeing the changes it's very possible that Facebook could still be testing things with a plan to roll the changes out on a wider basis in the future, though when that would be exactly is unclear as Facebook hasn't mentioned anything about these changes. Also worth noting is that given the fact these changes aren't really that big, Facebook may not actually announce them and just roll them out.