This is so annoying because I thought deleting the messages from the hub will only delete them from the BB and not from my Facebook account inbox. My BB Q10 runs on Blackberry 10 OS 10.2.0 which doesn't have any option to control how messages are deleted on the device and messaging server. I guess this is now possible with the "Delete ON" Blackkberry Hub option featured in Blackberry 10 OS version 10.2.1.
While clicking all the links in my Facebook Inbox, I discovered that the missing messages were "Archived". I was so happy when I discovered this. Instantly, I "Unarchived" all the messages and they were all restored back to the inbox. :)
So, if you want to retrieve deleted Facebook messages, you can click on the "More" > "Archived" link via your Facebook Inbox (ON PC). Beside each archived message is the "unarchive" icon which you can click to restore the message to your Facebook inbox.
NOTE: You cannot retrieve messages that you've permanently deleted.
I hope this helps.
Kindly share with Friends especially on Facebook.