John C Abell is the New York Bureau Chief for Wired.com and edits the Epicenter Blog. The opinions expressed are his own.

Facebook is the 800-pound gorilla in the social media space, with some 200 million members, a valuation of perhaps $5 billion and a base that has expanded well beyond its early roots as a private hangout for bored Ivy League students.

But, like the ad says, life comes at you fast — and there is nothing more unforgiving than internet time. So, are the best years ahead for Facebook, or is the finicky mob of cool kids — and now their parents and grandparents — already peering down the road for another Next Great Thing?

One thing is for sure: Nothing lasts forever. We need Microsoft, perhaps, but nobody gets very excited about it anymore (except maybe a hyperactive Steve Ballmer). AOL? They could do no wrong when dial-up was king. But when broadband made competitors out of the telcos AOL had leveraged to create an onramp to the internet, a steady decline into near oblivion began.

The internet roadside is littered with bold ideas hatched too early or denied a dignified death. David Wetherell was considered a bold genius when, in 1999, he turned up his nose at a deal that would have valued his internet incubator CMGI at about $18 billion. Fast Forward to 2009: CMGI is now ModusLink Global Solutions, and worth about 1/100th that.

But at least it’s still around. Netscape, the granddaddy of all dotcom IPOs, was purchased by AOL before the dotcom bubble burst and was RIP for all intents and purposes in 2007. Video powerhouse Broadcast.com? Bought by Yahoo and now ghostware. Yahoo isn’t doing all that great, for that matter.

So far, though, Facebook keeps catching the wave. It not only makes money, but a profit — perhaps as much as $200 million this year. Not bad for a company that has no discernible business model. It is said to be worth as much as $5 billion, a figure extrapolated from rejected cash infusion offers by private equity firms and the secondary market for employee insider shares. That’s a far cry from the $15 billion valuation implied by Microsoft’s $240 million investment for a 1.6 percent stake in October 2007, but it ain’t hay, and we are in a global recession.

Valuation is an important part of the Facebook story, but other numbers are even more important: Facebook’s demographics. The story of this metric can almost be told with a single factoid: Mark Zuckerberg matched the median age of a Facebook member when he started the thing — and now, at 25, he still does.

Age creep is a good thing — the alternative is death, remember. The college farm system delivers basically the same number of students at any given time, and they are always about the same age. Growing your membership would mean skewing the metric up or down. It’s hard to go down too much. As it happens the fastest-growing group is women over 55, and there are even slightly more members between the ages of 45 and 65 than there are 13-to 17-year-olds.

So far Facebook has avoided the worst mistakes of other social networks, which contributed to their own irrelevance by failing to adapt. It might have been the death knell for Facebook to open up to the parents and grandparents of their earliest adopters, but it didn’t turn out that way. Facebook has attracted a vibrant developer community, but they don’t allow the rampant customization that makes some MySpace pages epilepsy inducing. And even though it has flirted with the dark side when it comes to user privacy, its support for OpenID may prove to be the most significant milestone in the advent of an open and social web.

Even so, Facebook must be hearing footsteps from Twitter, the new kid in town. Twitter is in many ways the anti-Facebook: a distributed society that does one simple thing rather than a gated community with a gazillion avenues and possibilities. Twitter became a geek darling at the 2007 SouthBySouthWest conference in Austin, Texas, and went vertical earlier this year when politicians and celebrities “discovered” it. Some of the cool kids are already saying it has jumped the shark now that the likes of Oprah Winfrey and Sen. John McCain on are it — but they are probably just jealous about the hundreds of thousands of followers these celebrities have.

For the moment, Twitter seems to be sucking all the air out of the room. In the “fastest-growing” metric it is as much a no contest in Twitter’s favor — quadrupling to 17 million U.S. visitors in the past two months — as it is in Facebook’s for overall membership. Like Facebook, Twitter has no real business model yet and yet seems able to sustain itself just fine for many years to come. On those rare occasions Twitter gets bad press, as it did last week when it changed an arcane user setting without warning, the worst insult hurled its way was that its imperious behavior was Facebook-like.

Even Facebook seems to get it. It rolled out design changes two months ago in a clear nod to the Twitter view of the world. As TechCrunch’s Erick Schonfeld put it: “Despite its already considerable size, Facebook is showing how adept it can be in responding to new threats. If Facebook cannot buy Twitter, it will try to beat it instead.”

Facebook has already made many transitions that have tripped up less nimble organizations. It went from being as exclusive as you could possibly be — Harvard students only — to letting in anyone. It’s as popular now with grandparents as it is with their grandchildren. It is a daily fixture in the lives of tens of millions of people.

Game over? Dude, this is the internet.

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