From Rodrigo Schmidt at the Facebook Blog:
Many of you have told us that sometimes your Chat session comes and goes or even stops completely. We’re working hard to end those interruptions so that your experience is stable and consistent.
The good news: We’ve already made progress, and we’re taking some more big steps in the coming weeks. For example, in the past couple of weeks, we’ve already made Chat faster and more stable, fixed bugs and improved the technology on which it runs.