The Flexhub testhub has been running for a while now, and looking stable.
A real-life test with over 500 NMDC users without any traffic limits has run successfully. With some optimizations I think the hub will be able to run well up to about 1000 users. When additional processing threads can be added after beta, and run stable, that number will be higher.
Join
dchub://test.flexhub.org:8000 or
adc://test.flexhub.org:8000 to see for yourself.
Warning: the ADC part isn't finished yet, joining and chatting shouldn't be a problem though.