Data transfer status exceeded

I wonder what caused this…

From: civics@geocities.com
Subject: Your Data Transfer Status

Congratulations, your website seems to be very popular and has been receiving a large amount of traffic. Our records indicate that you’re using more than the allotted amount of data transfer we provide for a free web site, which is 3 gigabytes per month (measured on an hourly basis). That means that during the past few days we had to temporarily turn your site off to keep the bandwidth within this limit.