For years, many industry observers including me have stated that email is the ultimate social network.

If you take all the people I email with regularly, you'll quickly figure out my top relationships. Then you can build a social network out of that.

And that's exactly what Google did with Buzz. But what I learned from that experience is that email is not one social network, it is many. And it may be a mistake to combine them all together into one network.

I'm writing this on a plane on my blackberry. I can't easily link to to Miguel Heft's piece in today's NY Times, but I can quote from it. He says:

"Email, as it turns out, can hold many secrets, from the names of personal physicians and illicit lovers to the identities of whistle blowers and antigovernment activists."

That was my experience with Buzz too. Many of the people that Buzz suggested I follow are people that I would never think of following on Twitter or friending on Facebook.

One of the more recent lessons for me in the world of social media is the value of the explicit connection. At one time, I thought that automatically building my networks would be useful.

But if you compare Google Latitude, where you broadcast your location, to Foursquare, where you explicitly checkin to a location, I think you'll agree that the explicit gesture is better.

And so it turns out that implicity deriving social relationships is tricky and potentially dangerous. That doesn't mean the idea isn't powerful. So was Facebook's Beacon. It just means you have to roll it out very carefully.

(For more from Fred, visit his blog)