If my memory serves me well, their Messenger was once XMPP-based (perhaps still is somewhere beneath) and you could use it without a fuss in any Jabber client.

If my memory serves me well, their Messenger was once XMPP-based (perhaps still is somewhere beneath) and you could use it without a fuss in any Jabber client. But then smartphones bloomed and the possibilities having a mobile app arrived. So, I'd expect same scenario here: open at the beginning and walled later - once they capture enough people within. That is, if this thing ever manages to gain attention.