Naossano that's a good line of logic, in fact I've already got something like that in the system and I'm just fine tuning it to make sure its a reliable count. Balancing the faction numbers is a problem I've thought about for six months and still have not a fool proof solution, only something that can help shape the team balancing a bit.
I'm not a big fan of hard balancing the faction numbers but it is common that team stacking occurs.
Currently, I'm still optimistic and implementing soft balance mechanics such as benefits for the underdogs and penalties for overcrowding factions. I'm slowly training my algorithm in detecting what an "ACTIVE" player is. Your reputation has a lot to do with it.