I would log into My Verizon and check your usage. I don't see how a FB notification would be charged as a text.
I know in my Hub, I can show just texts, FB messages, etc. Let me know what you find out.
If you added your Facebook account to your Q10, messages will show up in your hub by default. But they should not be text messages, as it's just a unified inbox. Check your Facebook account from your computer and make sure mobile text is disabled. Also, try StreetDocRN's filtering tip if you don't want to see Facebook messages in the hub.