It is true that Reddit does not hold a valid license to content that is
- Sufficiently long-form, unique etc. to be copyrightable, and
- posted by someone other than the copyright holder or someone with a sufficient license.
However, as far as I understand it, the extent to which Reddit—a content provider and social network—is legally required to remedy this is to comply with DMCA requests and review reported content. Perhaps there is a higher standard that I am not aware of?