That's a good point. Just like back when the iPad first got voice dictation but not Siri per se...Hmmm, the microphone icon is functional for me in both the messenger app and the Facebook app.
Edit: I don't consider the microphone icon to be part of Siri. I think it's simply voice dictation since you're not really interacting with Siri herself. Just my interpretation though. I get the microphone icon when using any app where the keyboard pops up, so I don't think it's a matter of individual apps supporting Siri.