No, your eyes don't deceive you. That's a plunge to exactly zero concurrent users that appeared this morning on Steam's active users graph. Gamers are currently unable to log into their accounts, and even the Steam mobile app seems to be on the fritz.
Don't panic: your account wasn't hacked. But the nature of the problem still isn't clear. On Steam's official Facebook page , Valve merely explains "The routine maintenance on Steam servers this morning is taking longer than expected. We appreciate your patience."
Like most mysteries of the universe, Valve's proclamation is fodder for subjective interpretation. Was the subtle injection of dry humor intentional? What would precipitate an extended maintenance window? We can cycle through typical causes -- a clumsy-intern-and-wire mishap, a sleep-deprived engineer, or an impromptu social experiment from Gabe Newell -- but the mystery lingers.
Curious. We've contacted Valve for more info which we'll hopefully receive unless its employees are battling back an infected horde at the Steam server rooms.
UPDATE: Aaaaaand... it's back!