The latest unc0ver 3.0.0.0 jailbreak (beta 46) can jailbreak and install Cydia on all iPhones and iPads that are running iOS 11 to iOS 12.1.2 and that use a Pre A12 SoC
You can now download Facebook videos directly on your iPhone, iPad or iPod using the jailbreak tweak "Prenesi" which is available on Cydia for free....