Yes, JioPhone does have a 3.5 mm headphone jack.
Yes, JioPhone comes with in-built Facebook Lite Application.
The standard packaging of the JioPhone includes Mobile Phone, Battery, Charger and Quick User Guide. You will not get earphone inside the box.
JioPhones comes with 4GB Internal Memory - so just like a normal Android smartphone, you can save as many contacts depending on the free space in your phone. In practial sense, you can store unlimited contacts.