Facebook is currently running a test that shows people when their friends are actually typing a comment to their post. This way, you know when someone is responding even before they finish their response.
TheNextWeb confirmed the test with the company, which gave them the following statement:
“News Feed is a place to stay up to date with your friends and family. We’ve heard from people that they want their experience on Facebook to reflect the real time conversations they have. So we are testing adding a line above the comment box of a post that indicates when a friend is typing. This is to help people have real-time conversations with friends and family.”
This is pretty much how Facebook Messenger already works. When you're having a conversation with someone, you can see when they're typing. It can help let you know if you need to stick around in the dialog for a second or move along to something else. It's pretty useful in that sense.
On the other hand, is this something we really need when it comes to commenting on a post? Facebook posts aren't quite the real time experience that a private discussion is. Various people will see a post throughout the course of the day as they scroll through their News Feed.
Some people don't really like the idea of people knowing when they're typing a comment, and find it "creepy".
People do begin comments and then change their minds. This must happen all the time. Now, you have to worry about looking like an ass in such a case.
It remains to be seen if this will become a standard feature. Time will tell.