i suggest to have a feature of TAGGING of usernames in posts.
that way if you refer one member in your posts, that member
will be notified that he was tagged (sent email)
therefore it creates and invites more posts, thus more traffic and interactive to the XF.
forums weakness really is the abilility to notify a specific member if he was the
topic of the hour in a specific thread, unless somebody will inform him
TAGGING like FB is a good feature for XF