Facebook has added a new Photo Sync option to its mobile apps, which automatically uploads photos taken on your phone to a private folder on Facebook. While that's potentially a convenient way of ensuring your current photos are easily available for sharing, it could chew through a lot of data if you're a frequent photographer.
The option isn't switched on by default, but once enabled it will automatically upload photos over available networks. The app defaults to smaller upload sizes when using 3G, but if you don't have a generous data allowance either disabling it altogether or only allowing sync over Wi-Fi would make more sense. Hit Facebook's help post (linked below) for detailed instructions on how to set those options in both apps. Personally, I prefer to stick to only uploading photos I have consciously selected, but this option could be useful for more regular photo sharers.