All URLs are URIs (as other people have noted), but the distinction is less important as time goes on. I think W3C's "URIs, URLs, and URNs: Clarifications and

All URLs are URIs (as other people have noted), but the distinction is less important as time goes on. I think W3C's "URIs, URLs, and URNs: Clarifications and Recommendations 1.0"[0] does a good job explaining the reasons why. Additionally, it mentions the proposed, but never accepted, Universal Resource Citation (URC). Sounds like just what social media needs these days! [0] https://www.w3.org/TR/uri-clarification/#classical https://www.w3.org/TR/uri-clarification/#classical