Weird decision. Judging by my logs, and those of several other sites I know about, it still has quite significant usage, though I don't know what their threshold is. For example, my own blog has 12x as many Google Reader subscribers as NewsBlur subscribers. Perhaps even with significant usage, that usage wasn't monetizable, and the what-people-read data wasn't valuable enough to keep it operating?

Good news for the competition, anyway: Google Reader being pretty good yet completely free and not (obviously) monetized occupied a lot of that space, which is now freed up.

edit: In fact, I see NewsBlur is completely unresponsive now, presumably due to sudden interest.