Over the past month two of my Facebook friends have had their accounts hacked (that I know of). In both of these cases the hacker chose to go through the person's Facebook emails to read all of their messages and then use that information to message their friends – posing as them. I was one of those people that each hacker messaged. It was a sobering experience to realize that someone could potentially have access to all of your personal discussions that you've had with family and friends, going back for years!
After this experience I decided that I wanted to delete all of my past Facebook messages so that if I was ever hacked, at least I could limit what they would have access to. But I discovered that this wasn't as easy as you would think as there is no global "Delete All" option for Facebook messages and conversations. Instead, you must click into each conversation, click on the "Actions" button, and choose "Delete Conversation". To do this for the hundreds of conversations that I've had on Facebook over the past several years would have been far too time-consuming. However, I did find an amazing plug-in for the Chrome browser called Facebook – Delete All Messages that did the trick.
The process is simple. Open up your Chrome browser and navigate to the plugin. Click "Add to Chrome" in order to download/install the plugin. It currently has over 82,000 users and I found it to be virus/bug-free as of two weeks ago, so I can highly recommend it.
Once it's installed, simply sign into Facebook. Open up the full messages window and click on the plugin icon that sits on the far right of the address bar.
Then click "Launch". It will delete all the messages on that entire page. If you have a lot of old messages you may need to scroll down and click Launch a few times until they're all gone.
But within a few minutes you should be rid of all your old conversations, and they will not reappear when the person messages you again. You'll have a clean slate!