I use Mnesia mostly to store temporary distributed data. If you want to store persistent data with another database you can just replace the implementation module for libsave with your own. libsave is my library for storing objects to disk.

So far I don't know how many users my servers scales to, with some rough test I can have about 2000 players in the same spot, I recently added a quad tree and with that in place I hope that will increase a lot if players are spread out in the area.