We talk about Facebook, twitter, MySpace and Friendster as the big social networks but we keep forgetting one that is far bigger than that: Skype. This from a Bloomberg piece on Skype’s vacillating fortunes:

Skype has soared in popularity since it started in 2003 and has about 548 million users worldwide—more than Facebook, MySpace and Twitter combined.

Pretty much everyone I know is on Skype—more so than Facebook—and their investment in it is greater: They had to figure out how to install software, set up a microphone, a webcam, create an account, and maybe even buy credit. More importantly, they can actually estimate its value to them, by counting the money it’s saved them, if they want.

We all know about eBay’s missteps with Skype over the past few years and the software could definitely do with a total overhaul. But now there are new faces involved—including Marc Andreessen, who knows a thing or two—I foresee huge opportunities ahead.

One is a route they’re clearly going to take: the enterprise. That makes sense, but it also means damping down Skype’s huge social reputation, since companies will tend to think of it as at best a frivolous time waster for its employees, at worst a security threat.

Still, it would make lots of sense to go that route, possibly creating a separate sub brand of Skype that built a wall between the existing network of users and the enterprise one.

But I think there’s a much bigger opportunity out there, one that was talked up back in 2005 but never left the ground. That was leveraging the free connectivity to allow an eco system of services to develop atop of it.

Consulting, translation, education, all that kind of thing.

This never really took off, but I think that may have had more to do with its execution, and the fact that the world wasn’t quite ready. Most people signed up to Skype for the free calls. They weren’t really interested in more than that.

And yet since then Facebook and other social networks have. (Taken off, I mean.) Doing, actually, pretty much the same thing. Setting up an account, adding your buddies to it, and then communicating.

But the potential of that network was never exploited. A few memory-hogging applications and a few desultory ads have been pretty much it.

Maybe now Skype can make the most of this. One is the eco system of services I mentioned, but there are also location-based opportunities, mobile opportunities, video opportunities.

If Skype dovetailed with Facebook, twitter and LinkedIn it could position itself at the heart of social media. After all, it’s probably the only application that most Internet users have installed, loaded and active on their computer. Unlike Facebook et al, Skype is there, right in the moment. It’s the ultimate presence app.

Indeed, it’s much more like an instant Rolodex (remember those?) than all the other networking services we use. If I want to contact someone the first place I check is Skype—if they’re online, what’s the point of contacting them any other way?

In other words, Skype offers a granularity that other social networking tools don’t: Not only is it comfortable with one to all (the status update message), it’s also comfortable with the one to several (add people to a chat or call), it’s also great at instantly connecting one on one. You can even reach people offline via it, if they have call forwarding enable, or you have their SMS details stored.

No other social network offers that.

Of course, Skype has some ways to go to do this. The interface needs a serious rethink: It looks so 2000s.

It needs to add—or reintroduce—lots of features, like individual invisibility (being invisible to some people and not others), to encourage those who either don’t have it running or have themselves permanently invisible, to keep it there in their system tray.

It needs to lower some of its walls to allow interoperability with other chat clients, like Google Talk, and with services like Facebook and LinkedIn. Indeed it should throw open all its doors, so I can look up my friends on the Skype app and communicate with them using any or all of those services. Skype is the app is the network.

Then we might be back to those heady days of 2004-2005 when Skype looked like it was not just going to be the end of ruinous IDD phone monopolies, but that it might herald a new era of networking.