It is illogical to use a tool that protects against hackers, while obtaining that tool from a hacker. The person posting "Free keys" on Facebook is often the same person trying to breach your system.
The truth is harsh: Any key you find is either already dead, will die within 24 hours, or is a trap designed to install malware on your machine.
To use ESET NOD32 Antivirus, you need a valid license key. A license key is a unique code that activates the software and allows you to access its full features.