When delicious was created, there was no way to sync bookmarks via "the cloud" like browsers do now.
But Pinterst showed how the "social" side of "social bookmarking" should really be done.
I remember showing the original del.icio.us to my friends and them all saying "I don't get it." And then I said "instead of emailing everybody links, you can just post them here and we'll all get them automatically" and they went "Ah! Now I get it!"
Edit: Am I wrong?
Meaning, yes you in the past had to share links via email. Delicious solved that with both parties on Delicious.
Or they subscribed in a RSS reader.
With delicious From day one you bookmarklets that worked with anybrowser. And a kickass Mozilla sidebar thingy