I have a monthly quota on my DSL connection. Last couple days, I noticed huge bandwidth consumption on my account. Since the culprit could be any of a dozen devices/a family member streaming video or so, I didn't pay much attention to it, though I suspected something was wrong.
Come today. No one home but me. DSL account shows no network usage all throughout morning (when I was asleep), but starts spiking around the time I woke up and turned on my Mac. No heavy browsing, no downloads, no video streaming, no nothing. So it's impossible that I was using all the bandwidth. And well, now I know the culprit is my Mac. (Automatic software updates are turned off, by the way)
Long story short (not very short I see, sorry about that), I got Little Snitch and noticed that the total up/down traffic of Photo Stream is constantly increasing. iPhoto is closed. I haven't recently imported any large sum of images. Only the few occasional shots I took with my iPhone. Also, the latest images are on all my devices; iPhone, iPad, and iPhoto on my Mac.
So I'm not sure what exactly it is that Photo Stream is uploading/downloading continuously.
For now, I denied all Photo Stream outgoing traffic till I figure out what's wrong, and before it depletes my monthly quota. Though I have no proof, I suspect it was Photo Stream that was resposible for the heavy bandwidth usage these past few days too.