It means people will user Facebook messenger when they are not comfortable with the place they are in. If I'm at home, at my university or my cousins' I will chose Snapchat over Facebook messenger, because I'll feel free enough that I can talk and goof around with that app, in a way that is not possible with Facebook... yet! Also, I think there are people less boring than me who will not give a damn and will use Snapchat's Chat everyday, everywhere, and that's going to be awesome. As clichéd as it might sound, they really are changing the way we communicate.

If the newest update is anything to go by (auto sending image as soon as it is taken) FB messenger is trying to get away from that

I think he's referring to lot of people that spend their time mindlessly sifting through their smartphone in order to avoid eye contact with a stranger.

And also that if someone sees you "selfie"-ing you will likely feel silly, whereas Messenger isn't as publicly silly.