Yes, JioPhone comes with in-built Facebook Lite Application.
Yes, JioPhone does have a 3.5 mm headphone jack.
Yes, JioPhone is a dual sim mobile phone. You can use one 4G sim card and one 2G sim card.
We suggest JioFi is a better option. Because -
01) JioPhone is slightly cheaper than the standard price of JioFi.
02) You cannot make voice calls direct from the JioFi but you can make voice calls direct from JioPhone - either from the Jio sim card or your regular sim card.
03) JioPhone can serve as a perfect standby phone as it has dual sim card slot and a memory card slot also.
04) You can use JioPhone direct to watch youtube videos, listening FM radio while traveling but in case of JioFi direct usage is not possible. You have to connect the JioFi to your main device.
But JioFi has 2700 mAh battery while JioPhone has 2000 mAh battery capacity.