Discovery is becoming social. Searching on Google is good, but having your friends help you find what you're looking for is better.
This dovetails with what I've seen on gaming sites: when someone has a question, they don't come to the website to find a document they can read, they go to the site to find someone they can talk to. That's why they usually end up in the forums, when there might be high-quality editorial content sitting unused elsewhere on the site.
It's worth remembering: many of your visitors come to your site because they have a problem, and their preference is to connect with another person to find a solution. So enabling that connection is the fastest way to reward the visit and gain a repeat visitor.
How we share is changing. People under-share because they don't want to appear self-important. But your friends really do want to know what you're up to. Facebook and FriendFeed let your friends discover what you're doing on their terms, and encourage more sharing, since you don't have to get in your friends' faces every time you update.
When you start to think of your web community as composed of friends, it changes the way you view web features. How do friends connect with each other? How do they want to interact? Do you provide them with the opportunity to connect in those ways, or will they have to go somewhere else to fill that need? I can think of dozens of "community" websites, but very few that allow me to connect with other members as if they were actually my friends.
So, as a mental exercise, imagine a scenario in which you and a friend of yours would need to keep in contact by way of the web. What would you need? Messaging, certainly, so you can keep in touch. And some sort of status report, so your friend could check up on you and see what you've been doing (those parts of yourself that you're willing to share, at least). There should be some means of scheduling get-togethers, whether online or off. And then a means of sharing items, be they photos or whatever. It's a rare site that enables more than one or two of those activities. How does your site compare?
Social sites? No, social Web. The idea of a site built around user content (like Epinions) is old-school. Today, users expect all sites to be social. They expect that if you're on a commerce site that you know your friends are also on, you can see what your friends bought there and if they liked it. Social is a feature, not a destination.
This is an idea that's good on paper, but very difficult to realize. There's a simple reason: compare the number of friends you have to the number of websites you visit. That's not a very good ratio, is it? Let's limit the discussion to just Amazon: when you're browsing books there, how likely is it that a "friend" -- no matter how tenuous the connection -- is likely to have browsed the same books and left recommendations behind? Even worse, think of all the books and movies you've bought and rented over the years; are there any titles in that list that you don't want your friends to know about (guilty pleasures, porn, or what have you)? Some people are compulsive recommenders (in The Tipping Point, they're called "mavens"); the rest of us will be too lazy and occasionally unwilling to share that part of ourselves.
So while, in theory, I like the idea of "social" being a feature, the reason that there are social destinations is because they're like the corner bar, which you visit because you know your friends are there. It's natural for Google to want community to be a feature rather than a destination -- they're currently promoting a technology for cross-site community features -- but the reality isn't there yet and possibly might never be there.