People are invested in Facebook with their updates, photos etc. Unlike what the OP contends, people are not going to switch over their usage to a competitor based on features. Bing is as good as Google for most queries, but how many people have switched to Bing? Google is not attacking, it is defending. FB messages is already way more efficient for quick communication than email. It may not replace email, but can occupy significant market-share.

No matter how many places Google pushes G+, people are not going to switch unless they can achieve the same functionality by copy-pasting a link.