Since you can't actually delete the message, do the steps 1 & 2 IMMEDIATELY instead.
Step 1: Change your Facebook name.
Step 2: Block the person you sent the message to.
Step 3: Make a new Facebook account with your old Facebook name.
Step 4.1: Use automatic unfriender.
Step 4.2: Deactivate or delete your old Facebook account
1. Change your Facebook name.
Account Settings -> Name
2. Block the person you sent the message to.
Your profile picture to that boss's friend's profile picture: it would appear blank to your boss (in his inbox) once you've blocked him.
The following steps are optional, and should be done if you think your boss would put lots of time and effort into figuring out who wrote the "I hate you" message:
3. Make a new Facebook account with your old Facebook name and looks exactly like your old account. So, when your boss looks up your name, he finds this duplicate account. Make make things private (only me) using privacy settings OR make an effort to make a full duplicate of your original Facebook account by adding all your previous friends and liking all your previous likes.
Choose 4.1 or 4.2 (If you changed your old account's name to Pineapple, using 4.1 or 4.2 people can't find out who Pineapple is)
4.1 Use automatic unfriender (Ninja Chrome App) (click here to go directly to the Chrome web app site) to unfriend all your Facebook friends on your old account.
Change privacy settings on your old account to make everything "Only Me" (or as private as possible)
(I've used that app before, it's great, though you might want to try free apps (this one needs Greasemonkey [Firefox] or Tampermonkey [Chrome]. If you're in urgent situation, don't waste time learning how to use the free app, that might take about 1-3 hours, and after all that hard work it probably won't even work! In comparison, the Ninja Chrome App is reliable and works about instantly (it should take you like 6 minutes to purchase it and set it up). The point of the Ninja Chrome app is that instead of you wasting time unfriending 5000 friends, it does it automatically.)
4.2. Deactivate (or delete) your old facebook account. (FASTER OPTION)
If you deactivated your account, then everytime after you log in to the old Facebook account, instead of logging out you should deactivate your account again (so that people can't find out who Pineapple is).
It takes 14 days for your account to delete, but it might be faster if you email them. Immediately after you choose to delete your Facebook account, your account will be deactivated for 14 days, and then deleted after those 14 days.
To deactivate your account:
- Click the account menu at the top right of any Facebook page
- Select Account Settings
- Click Security in the left-hand column
- Click Deactivate your accountWhen you deactivate your account, your timeline and all information associated with it disappears from Facebook immediately. People on Facebook will not be able to search for you or view any of your information. - quoted from How do I deactivate my account? - Facebook