After trying Stratus and FMS, we've decided to use SFS Pro and RedBox to create a 1-on-1 random video chat between Facebook users (similar to chatroulette, minus the porn, hopefully...)
I created a massive sfs extension that handles the random search algorithm, friends invitations and chat messaging, and use 4 red5 servers to stream video (each user connects to a random red5, and sends its IP as user variable).
In the meantime we've reached a peak of 4500 concurrent users on the SFS server, without any load on the server at all! (0% cpu), very little to no dropped messages.
Very good work, SFS team !