Campfire was released in 2006, 67 months ago.
60,000,000 messages, over 67 months, works out at 895,000 a month - approx 28,000 messages a day.
Or one message every 3 seconds. Campfire could be run from a single small VPS.
I think it's entirely believable that the usage of campfire is that low. All credit to them, they've managed to get people to pay for something people don't actually use very often.