Facebook is reportedly getting ready to test a feature that will let you start private messages to friends from the main status update box on the homepage.
Josh Constine at TechCrunch says he has confirmed with multiple sources that this is going to happen, starting with a small percentage of users. He writes:
Users will be able to switch between posting to the news feed and privately communicating with select friends.
We haven't seen any screenshots of what this will look like exactly, so it's hard to say how easy it will be to accidentally post private messages to all of your friends, but if this is out there in testing, you might want to pay extra attention to what the text around the box says before you push "post".
Google+ pretty much already operates this way. You can easily switch from "Public" to any number of select people, effectively turning communications into private conversations.
Google also recently did more to combine its various messaging products under one "Hangouts" banner.