Hangouts supports xmpp clients (but not federation), so it's similar to facebook afaik.

No, if you have a client connecting to Google's XMPP servers, your messages will not show up if the other user is only logged in on Hangouts. Hangouts ALSO can't send messages to an XMPP only user (even if Google is hosting the talk servers, as is the case for my Google Apps domains).

Did you try it? (that doesn't match what other people have explained to me)

Yes, I did try it.