How to delete App Documents and Data for iOS apps like Twitter?
Actually, most of that data is cached from the internet for the specific apps you mention. Twitter and Instagram cache images, tweets, posts, etc. from the Internet. There is currently no easy way to remove them, however, you can send feedback to Apple here, and tell them you want a way to delete app cache.

You can restore from iTunes to remove it, as it's not backed up. If you run out of space on your device, iOS will automatically remove the cache.