No, in the App World summary of the Facebook application it says version 2.0 is available for OS 5 and higher and that version 1.9 is available for OS 4.5-4.7. You would need to look into getting a seperate chat application that's compatible with 4.6.
I think yes. Because my aunt has OS 4.6 and she said she needed to downgrade from the latest fb version to the previous one. When I check it really does say os 5. but I wanted to be sure. And check rroyy's site for OTA link and made my aunt download it. And she said it worked perfectly.