Yes, JioPhone does have support for Facebook. It comes with Facebook Lite App.
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.
Yes, JioPhone is a dual sim mobile phone. You can use one 4G sim card and one 2G sim card.
Yes, You can type in Malayalam language in JioPhone. JioPhone supports 22 Indian vernacular languges.