hello and welcome.
Facebook Engineer Eugene Letuchy tells us:
“The secret for going from zero to seventy million users overnight is to avoid doing it all in one fell swoop. We chose to simulate the impact of many real users hitting many machines by means of a "dark launch" period in which Facebook pages would make connections to the chat servers, query for presence information and simulate message sends without a single UI element drawn on the page. With the "dark launch" bugs fixed, we hope that you enjoy Facebook Chat now that the UI lights have been turned on.”
Dark Launch is fitting.
I hope to share here what I've gathered from many hours searching the net.