Did all the posters you mention "defect" from here to there? If they want to post over there as well, good for them, but I'd be very surprised if they stopped posting here.
Most of the members who have moved to that forum are still active on this forum in fact their activity on this forum is way more on this forum than on that forum.
All of the members have to allocate time on daily basis to post on both forums. A good number of users have moved to that forum, however nothing is wrong if the members can allocate more out of their daily time on another forum.
If someone is ready to put more effort on another forum and also has an account on this forum then that should not be a general issue I guess. The managers also shifted to that forum but they're still more active on this forum.