The worst thing you can do to an introvert - point out that they're socializing.
Like, what's the response you're expecting when you do that? "Yeah, I'm talking now, my introverdness has been cured, God truly does exist!"
No, they're just gonna feel embarrassed for being put on the spot like that and go back to their emotional shell!
Don't ever tell an introvert "hey, you're talking now" or "hey, look who's come out of their room".
Just let things flow naturally.