Xmpp via the jabber app was growing organically as a cross platform chat app with first mover advantage. My take is that google stood on the shoulders of the open source giants and used the early adopter community as a free quality and assurance team for Google talk until they couldn’t take any more value (users or q&a volunteering). It is possible that jabber could have declining the whole time but the fact that it still exists and has lasted longer than apps killed by Google means that there is more evidence of it not needing Google.