Therebirth's population has risen alot in the last few months, it's around 700-800 now.
It's also based on some sort of cloud service, which according to them, should make it impossible for stuff such as rollbacks and delay to occur. That's pretty sweet to me and something I enjoyed leveling my mage back when I played on the server.
Oh and everyone isn't running around in T2. It's pretty cool when t1 and blue PvP set is considered end-game stuff.