Dave Winer, on Our Social Web:
"Support for links is the basic requirement of the web, the same way we say feeds are required to be a podcast. If you don't support links not only aren't you the web, you're anti-web."
I can't think of a more basic requirement for the Web, let alone the "social web". I'd add that it has to be two-way linking, as well—both links in and out. Instagram, in particular, was always a non-starter for me in large part due to this.