Facebook is revolutionizing communications with one very simple feature, the "Like reply." Maybe "revolutionizing" is a bit much, but the new feature is actually quite useful.
In Facebook Messenger, users now have the option of replying to something with a thumbs up, aka the Like button icon.
By default, before you type anything, the Send button is replaced by a like button. This way, you can simply hit that to reply to something you'd otherwise just type "k" in response.
Agreeing to something is one of the most common messages you send, in any type of conversation, but especially in a chat.
On mobile devices in particular, where it pays to keep it brief, the new feature should come in rather handy.
Facebook added the feature to a recent update to Messenger for Android, iOS, and the web. It's important to note though that, like most of Facebook's new products and features these days, the "Like reply" is a blatant rip-off from Path, which has had the feature for a few months.