There are two popular online concepts — ones I never thought much of — that, when combined, may add up to one of the most powerful mechanisms on the Internet, well worth following. I'm actually amazed at how two dogs can produce a winner.

The first concept is the social network. I've written about these before and have joined a number of them, including LinkedIn, Orkut and others.

Many are set up for socializing, such as Friendster. Others, such as LinkedIn, are for making business contacts, although my experience is that these ideas sound better on paper than they do in the real world.

The second online concept I'm referring to is collected public reviews. Epinions may be the leader in grass-roots reviews, but you tend to find things like reviews of your local restaurants, where people who run the restaurants post endless commentary. Amazon probably popularized the idea of shared reviews with its public book reviews.

The problem with all these reviews — including Amazon's — is that they are often written by either shills or people holding a grudge, each providing the public with BS ideas from every angle. This makes the entire process completely useless.

Enter Yelp!. Here is one of the most distinctive ideas I've seen in a while, although it is nothing more than public reviews combined with a social network.

But when you combine these two concepts, you actually end up getting useful information, because the people participating are more than just anonymous reviewers from who knows where.

They are real people expressing their tastes and philosophies, and their collective wisdom becomes unlike anything I've ever seen on the Internet — at least anything in the territory I'm describing.

Here is what happens at Yelp: Say you are looking for a restaurant nearby. You can ask for a vicinity search based on a ZIP code.

All the reviews for local restaurants appear and you select a few that sound interesting. You can then click on any given person's network of friends and get his or her reviews. It's easy to get a clue about where any particular person is coming from based on the reviews.

What you get is the gestalt, if you will. If it turns out that a person likes Denny's and hates Le Bec-Fin in Philadelphia, then that person is probably not someone who would be great at suggesting fabulous French restaurants, but he might be good at finding a coffee shop.

The way Yelp works is very much like the various computer-generated suggestion systems designed to determine that if you like A, B, and C, then you'll probably like D, E, and F, since everyone else who liked A, B, and C liked D, E, and F.

The information on Yelp could easily be used for this sort of automated, computer-aided analysis, but it's actually more fun just to do your own searches manually. By doing them manually, you kind of get to know people and gain some feeling for their tastes.

The combination of computerized social networking and online opinions is powerful and useful. The combination also must be promising for people in the dating scene.

When stepping back to consider the Yelp system as it now exists, there are a few points to note.

First, the entire network seems to be populated by mostly Gen-Y yuppies, along with a lot of weird slackers, metalheads and a few odd politicos. There are also quite a few tattoo mavens in the mix.

It's actually fascinating for anyone over 35 to roam around the database. Marketing folks would have a field day!

Given the age of most people on Yelp, I'm surprised by how many of them call themselves "foodies."

Although the site seems to lean towards restaurant reviews, many of the women, in particular, seem to be reviewing everything from the service at the local post offices to every imaginable spa and manicurist in the area.

I ran into one poster who must have reviewed 20 high-end restaurants and 50 spas. It was as if she'd get taken out to dinner and then need two or three treatments in between. T

he other thing that kind of amazed me was the overall verbosity of this crowd. They write lengthy essays about just about anything from the neighborhood grocery store to the sausage selections at a ballpark. It's quite stunning.

One interesting use of sites like this is for avoidance behavior. This is a variation on an old trick you pull with wine waiters in restaurants. Ask them casually about wines you have already tasted and know well to see if they know what they're talking about. If not, send them away.

This concept also works with Yelp. For example, on Yelp there is a review of Restaurant Gary Danko — probably the highest-rated restaurant in San Francisco at the moment, with universal 5-star reviews.

Some punker vegan chick (for want of a better description) gave Gary Danko's one star, and her review was a 1970s-style rant against foie gras and veal.

She went on and on about cruelty to animals, and I was wondering exactly why she went to an expensive French restaurant like this in the first place. Just to complain? Seemed odd.

Considering her idealism and attitude, I could look at all her other selections and discover all the places I want to avoid.

People of this ilk tend to be boorish and offensive in more ways than one. When you live in Northern California you run into too many of them, and it's unpleasant. Now, accidental encounters can be minimized, thanks to Yelp.

I'm not sure whether Yelp will ever scale enough to handle thousands of reviews of any single restaurant, or whether the mechanism can ever work forever, but right now it does.

All this said, I did join the Yelp group, posted a funny picture of myself, and within 24 hours I got my first message. You'll love what it was.

In my Yelp e-mail box was a Nigerian Scam letter! I guess there are still a few bugs in the system.

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