if you really need to scale in the region of many thousand concurrent connections you should not blindly trust some third opinion. You can have a look here http://www.igniterealtime.org/about/OpenfireScalability.pdf for Openfire scalibilty and by any means do your own tests before rolling out whatever you choose. Scalabilty is more than throwing the right software at it, you have to evaluate your hardware, network, OS and the surrounding services (Database, DNS, Filesystem etc.) carefully.