What's so "social" about Delicious? This is a serious question.
You can subscribe to bookmarks from other people (only if you already know their logins) and you can send bookmarks to others (using a weird for:nick meta-tag). Both features seem a bit sketchy by today's standards. This was definitely a breakthrough when Delicious was launched, but nowadays people expect more from a social site.
Not that I care about social features. I didn't miss them in Delicious, and I don't miss them in Pinboard. But it just doesn't sound convincing to brand Delicious as a great social product for Google to acquire.