The lowest of the low will be 5K, then all the serious bugs will be worked out, itll rise to about 7 again then something will break. Itll go back down to that 5k for the people leaving (either again or for the first tine) where as the few remaining players will be those who returned and are happy with the changes and the people who are here for the long haul.
In about 8 months there will be a HUGE game change that will bring the numbers back up to about 11-15k and then it will dwindle again until console release. Itll rise again and possibly even more than seen recently and then it will die down with the console focus.
This is just how these games work, its predictable.