Yet none spider the Web, nor create an index of content outside of what resides on their own respective databases. Also, none exist with the primary purpose of sending their visitor traffic away, as do Yahoo!, Google, Ask, and MSN.

These, and similar sites, also operate to some extent like directories. But in behind-the-scenes conversations with these companies’ founders and managers, you’re likely to hear some very interesting comments. “We’re the largest shopping search engine in the world,” an eBay executive once told me. And you thought it was an auction site! It’s no small wonder eBay acquired Craigslist, the website that’s long been giving traditional classified ad providers a run for their money (online as well as off). It’s also no surprise that Google snapped up YouTube, where people fritter away hours searching videos they’re interested in and “video snacking.”

Zillow, if you haven’t heard of it, is a powerful real estate search engine that not only provides listings of available properties in designated areas, but also all kinds of other sales and property valuation information. Like its better-known peers in the real estate listings vertical, Zillow is navigated primarily via search.

You might call this category of sites directories, specialized or vertical search engines, but for the purposes of optimizing content on them, it really doesn’t matter. As with the “real” search engines, you can buy ads on most of these websites. But what really matters to most of the sellers, buyers, browsers, and users of these sites, commercial or otherwise, is the ability to find what they’re searching for.

It’s easy for SEOs to get so caught up in optimizing their own sites that they neglect the extra effort and push it takes to optimize their off-site efforts. With search the de facto way to navigate the Web, not to mention many of the major sites on it, this is a grave oversight.

The reasons to represent products, brands, services, or other offerings on websites other than your own are manifold. The first and most obvious is it broadens reach. Companies may, for example, post open positions on their own websites, but the pool of potential candidates is obviously deeper when jobs are listed in classified ads in local newspaper sites or on Craigslist. Perhaps you sell products. Listings on eBay, or even an eBay or Amazon merchant store, can put your wares in front of plenty of additional eyeballs.

An off-site presence, whether through listings or advertising, can confer the added advantage of inbound links back to your site from sites carrying links back to yours. Some, such as Wikipedia, are highly valuable and sought-after. Wikipedia listings have begun to appear at or near the top of Google’s search results as the online encyclopedia grows in reach, influence, and credibility.

That’s why wise search optimizers don’t just focus on how their own web properties perform in major search engine rankings. They’re also careful to optimize their presence elsewhere as well.

The principles of off-site optimization don’t significantly differ from getting your own site ranked in search engines. The overriding goal of off-site optimization remains fundamentally the same as with search engine optimization: Get found when searchers are looking for what you’ve got. The potential bonus is adding a jolt or two of visibility by means of link juice to your own website or property.