Though it may be true that videos which show up on the Facebook main feed are pre-downloaded, I am skeptical that a half hour of browsing facebook could be responsible for breaking a standard monthly data plan. Maybe. But there may be other factors you could look at.

According to AT&T's data calculator:
http://www.att.com/att/datacalculator/
You should not burn through that much data in half an hour. If my math is correct, your data usage equals about 992 kilobytes per second over that half hour period. That seems a bit much unless you were watching Netflix or something.


So questions to ask:
- Are there other programs running in the background which are downloading data?
- Is there a file sharing program of some kind running, such as BitTorrent?
- Are there other browser tabs open in the background which are downloading data?
- Are operating system updates downloading in the background?
- Are you sure that someone else isn't using the wifi connection?
- Are you sure that the device you got isn't a used device which already had some data usage on it (are the timestamps of the data usage they gave you accurate, or do some fall outside your usage range)?
- Are you absolutely positively sure that your number of 1,785,699 kilobytes is actually in kilobytes? Or is it in some other measurement amount other than kilobytes?