Oh, so each individual app -has- to support Siri? You don't automatically get Siri whenever there is a text entry field, regardless which app it is, as an OS-level feature?
I don't know whether FB supports Siri or not. I wouldn't have used her for dictation into it anyways, but it caught me by surprise to see her icon greyed out, as I'd never seen it that way before...
I just checked whether posting a status update on FB can be done with Siri, and in fact it can.
At this point, it seems that the company's other app "Messenger" is the specific app where Siri's icon keeps getting greyed out.