I once used almost all of mine in one day when I opened Twitter for the first time and left it on for a couple hours. I have never opened Twitter again. Can also happen if you leave a Live cam open for an extended period.
I don't use either of those things. None of us use the wifi for our phones. And like I said, our internet was out for at least 3/4 of the last month. It's been terrible. I'm paying for service I'm not getting.