Yup, something like this! :)
Regarding the possibilities, I think we could facilitate Facebook application to do this. With Facebook apps, it could push notifications to the user, and with Facebook API we could integrate the Facebook app with Xenforo. The technical details need to be inspected...
So far, among other forum softwares, I have seen XenForo as the most integrated software with social media features.
However I notice something that I think could be used to improve XenForo social media integration.
That is, I haven't seen a Facebook intergration which let user to subscribe to...
Sorry, after I rethink, this thread seem to be posted in wrong place :/ New thread made > http://xenforo.com/community/threads/facebook-integration-receive-notification-to-new-replies-via-facebook.31007/