I concluded in a post in August that “maybe Twitter isn’t for everyone.” Twitter peaked in total reach that month, at least in terms of U.S. unique visitors, according to analytics services Compete and comScore (NSDQ: SCOR). Facebook, on the other hand, is for everyone – everyone who has friends or family, which happens to be everyone on the planet.

But it can’t be everything to everyone.

In particular, we should not conflate the fact of Facebook’s and Twitter’s dominance of social media with the reason for their dominance: the emergence of the stream as a media form. The stream as a media form is what is often lost in the narrative of the rise of Facebook and Twitter and the focus on the personalities that drove the success. As a media form, the stream will evolve away from the monolith it is today, into something differentiated, varied and dynamic.

Facebook is the broadcast television network of stream communication. It pioneered the stream as a media form and popularized it for a mass audience by connecting friends, classmates and family. Like NBC’s fuzzy black-and-white 1947 World Series, the stream of 10 years from now will look nothing like the stream of today.

Over the coming decade, at least two types of winners will emerge from the stream wars. The first set of winners will be the creators of proprietary, differentiated streams. Some of those companies will create their own programming, while others will rely on the second set of winners – production houses, the social-media equivalents of Sony (NYSE: SNE) Pictures and Warner Brothers – to create it for them. Companies like Zynga and Playfish are the modern-day production houses, churning out applications for the social networks.

Pessimists will say: “What is there to differentiate around? One-hundred-and-forty characters is 140 characters, a photo is a photo, a video is a video …” That may be true, but if you believe that the stream is an ongoing conversation, it follows that the conversations I’m willing to start with my friends and family differ dramatically from the conversations I’ll start in an effort to pick someone up in a bar, or trying to network at an industry conference, or chatting with my fellow sports fans. To put it simply, I don’t want my mom on Facebook reading my pickup lines, commenting on my status posts intended for colleagues, or liking my sports chatter – so I’ll do those things elsewhere.

The bottom line is that despite the privacy settings Facebook users may now have, they still mostly intend their communication for friends and family, not for everyone, and even if Facebook could somehow cross that chasm, the resulting user experience may well be impaired compared with a dedicated network powered by a thematic stream—one with affinity for something other than school, friends and family.

Twitter itself is proof that the stream is not a one-company affair. While it may well never rise to the level of dominance of Facebook, there is no reason to think it will not be a highly successful media company for a long time to come. It has differentiated around celebrity, simplicity, breaking news and openness. For me at least, it is the morning paper. Twitter, despite lacking the mass appeal of Facebook, continues to aggregate a tremendously valuable audience while having an important enough role to shape geopolitics.

We recently conducted a survey to poll users of Facebook, Twitter, MySpace and myYearbook on why they used the various services. We provided 12 options and asked the user to pick the three that mattered most to them. You can see in the charts below why Twitter has been able to differentiate from Facebook, while Myspace, which shares three of the four main reasons for using Facebook—including the main reason—has not.

So there are at least two ways forward for social media in a stream world – even in a stream world dominated today by Facebook and Twitter. You can dedicate yourself to creating applications that play well in the stream, or you can try to come up with a new way to shape the stream itself.

myYearbook pursues both. It differentiates its own stream, myYearbook Chatter, in a number of ways. Most of all, the people posting into Chatter have a different purpose for sharing than people posting into Facebook – they want to meet new people, to flirt and to connect with people both near them and half-a-world away. This context renders the user’s real-life social graph, and Facebook’s main advantage, irrelevant. Chatter is now a geostream with 700,000 posts per day and double-digit monthly growth rates.  It has become so important to the site that we are currently in the midst of a major site redesign to put Chatter at the core of the experience.

But to support the purpose of meeting new people, the stream needs to function differently than the standard Facebook stream. Our Chatter stream differs in two critical ways: 

—First, users view the chatter of people near them, filtered by age and gender, making Chatter, at its core, a geostream, and one well-suited to meeting new people.
—Second, unlike other popular streams, Chatter permits users to comment on other users’ posts with photos as well as text, opening the door to interactions like the one depicted below.

If we look just a couple of years down the road, there is no question that Facebook’s U.S. uniques will have peaked. Not for any flaw in the company’s model, but for the success of it – everyone will have signed up – so their growth rates will be checked by the population. At that point, churn will grow, as users discover other niche social networks, just as the broadcast television audience fragmented across so many different cable stations.

Meanwhile, Facebook revenues will spike as the dramatic increases in inventory fueled by growth start to slow and CPMs start to rise, putting intense pressure on the social-gaming companies that, in the face of increasingly restricted viral channels, have benefited from cheap advertising to make their high-churn businesses work. The winners will be those companies who grow virally—or, put another way, the companies that play well in the stream.