Since the middle of August, before I got my Fitbit but after my Android phone started tracking my steps, I've lost 7.6 kg, finally hitting my goal yesterday. This morning I was only slightly above the goal; and also for the last three weeks I've been barely creeping towards it; so I figure this might be permanent.
I only made two adjustments that I'm aware of: one, I pretty much stopped drinking beer in favor of other things; and two, I'm much more likely now to make detours that add walking distance to whatever I'm doing. I might have gotten more disciplined about food, but I don't think so, as the meals I eat now are pretty close to the meals I ate a year ago.
It does make sense that a small change can, over time, have an effect like this. So I think beer and the Fitbit probably did cause the weight loss.
Of course, if it turns out I have some horrible illness instead, I'll be very annoyed. But a 250-gram-per-week loss is more consistent with tweaks to diet and exercise than it is with, say, tapeworm.