Just found the TOS while browsing.
The link under "sign up" (leading to https://flat.io/legal) was the one I referred to as broken.
(...) you grant us a worldwide, non-exclusive,
royalty-free and perpetual licence to use,
copy, reproduce, distribute, adapt, re-format,
modify, publish, translate, licence, sub-licence,
and exploit the User Content anywhere and in any
form for the purposes of providing our Service (...)
@gnud Thanks for the link, we will fix this shortly. This part of the ToS applies to the public & shared content that can be accessible by everyone on the internet ;-)
As written, it applies to the "music, sounds, text or information that you make available or create whilst using the Service" and gives Flat a "worldwide, nonexclusive,
royalty-free and perpetual licence to use, copy, reproduce, distribute, adapt, reformat, modify, publish, translate, licence, sub-licence, and exploit the User Content
anywhere and in any form." [1]
I can see why artists might be a little concerned about this.
We will update them in next few days / weeks to be more precise about private and non-shared content. This actually more apply to public content (e.g. you can see on "popular" pages).
