Turns out any site that allows users to submit and retrieve data can be abused in the same way:
- FacebookDrive: "Store files as base64 facebook posts"
- TwitterDrive: "Store files as base64 tweets"
- SoundCloudDrive: "Store files as mp3 audio"
- WikipediaDrive: "Store files in wikipedia article histories"
See also https://github.com/qntm/base2048. "Base2048 is a binary encoding optimised for transmitting data through Twitter."