A challenging aspect of running a XF forum is getting back lost members. There is a simple way to solve this. For members who have not visited the site for over X time, show a 'Miss You' link / button on profile. Users on the site can then click this functions to let the member know that he/she is missed. The member will then receive an email to let him/her know that members, A, B, C misses them and would like to see them back. Of course with a few call to action links back to the site. Emails can be sent once a week or so. Members would not receive more than 1 email a week. No matter how many members click the button. Do not send out a 'We Miss You' notification from the same member until the absent member has visited the site again.