How do you send a blank message in facebook chat?

You type in " * * " [without quotes]

Whatever you type between two asterisks, will be bold. For example if you type in " *Facebook* ", the message will be "Facebook". So, since the bold version of space is the same, you end up sending a blank message. You can put as much of space between the asterisks as you want, the more you put, the larger the blank space.