All you need to know for this is just go through the all steps.
-> Login to your Twitter account.
-> Click on Compose a new Tweet.
-> Hold ALT and type 0173 from numeric keyboard(which is at right side of your keyboard).
-> Now click on tweet.
-> Take a look at your recent tweets and you can see that there is a blank tweet.
You can do the same trick on Facebook. From this way you can send blank massages, blank comment as well as update blank status too. So make fun with this trick with your follower and with others.