Steve Rubel raises a good question, as to whether online communities can ever "stick" for any length of time. Most don't last for long. He goes down a long laundry list--The Well, Tripod, GeoCities, Friendster and so on. Remember CompuServe? All had communities at one time. All came and went. Now it's even more competitive and complex, and the stakes are higher. What does this mean for the likes of a FaceBook or MySpace, let alone a smaller fry like Yelp?
He points out that only a handful of sites endure over a dozen years. Those sites all have "moats" that protect them, he says. "These barriers to entry include peer-to-peer commerce (in the case of Edelman client eBay), robust user reviews (Amazon.com) and deep entrenchment in vertical markets (BlackPlanet.com)."
Besides the moats, I'd also say at least in the case of Amazon, they know how to really serve their customer, uh I mean community visitor. Their user review system is the best. The sales process is automated and smooth, and of course, they use all of their intelligence to tailor choices to each "If you like XYZ book, you might consider ...."
So it may be mixing apples and oranges, but Amazon does have a community of sorts, mixed with a powerful e-commerce machine--and it works. By comparison, so far, as Facebook tries to overlay a new marketing system on top of its former community site (while sucking in millions of oldsters like me), it's messy at best. Complaints about violation of privacy run rampant.
My guess is that community sites come and go because that is the nature of communities,which are fleeting and fluid, not much different than past times, when neighbors would convene at the local coffee shop to trade gossip (and still do). Some of those last for years, but in other cases they break up after a few weeks or months. Facebook's juggling act is to try to keep people connected and coming back, while they try a slew of marketing schemes to milk everything they can out of this model. They are trying to defy gravity, in a sense, and break from the past. They'll probably pull it off, but don't bet the farm on it lasting forever. The ghosts of The Well and all of the others say otherwise.