With Ask.com introducing the AskEraser — a switch that will stop the site from collecting information about a user — it’s worth checking in on the real state of play with the accumulation of data online.

As usual, the reality is very far from the public perception. Ask is far down on the list of sites that anyone who cares about privacy would be concerned about. It is hardly pervasive, so it doesn’t collect much data at all. And Ask doesn’t even run its own advertising system (it uses Google) so it doesn’t have much reason to collect data.

Of course, Ask is simply trying to gain marketing points by differentiating itself from Google, which to some embodies the erosion of privacy in the Internet world.

Google indeed collects a lot of data. It sees the bulk of the searches on the Internet and an increasing amount of other activity. And it obsessively files away most every scrap of data it receives. (Google will say that much of this data doesn’t include the personal identity of the user it is tracking. In fact, it actually has enough pieces of information to identify a lot of users if it really wanted to.)

Google, however, has been very reluctant to use all this data in its advertising business. One reason is that it has other information that solves its main problem: picking the right ads to show on each page. It uses what people are searching for on its search site and the content of other pages on which ads appear (including, of course, the content of messages displayed in Gmail).

But as Google gets bigger it is tiptoeing into using more data for targeting. It tries to determine the location of users in order to show ads of local businesses. It also gets some personal information about users from partner sites on which it displays ads — like MySpace — to help it choose ads.

And Google has now started dipping its little toe into the pool that Madison Avenue calls behavioral targeting. That approach is based on the idea that the best way to pick an ad to show you now is to look at your online activity from a few hours or days ago. The classic example is showing car dealer ads to someone who searched for minivans yesterday.

Google is testing this concept by exploiting a feature of the way Internet browsers work, according to a Google spokesman. When a browser asks a site for a given page (such as the search results for a specific term) it sends it the address of the last page the user saw.

Google is using this information to take into account what you just searched for and your previous search when it displays ads. The Google spokesman said the cookies that Google places on users’ hard drives to identify repeat visitors do not come into play here.

Here’s how you can see this in action. Search for “lawyers” on Google. You will see law firm ads, some perhaps near where you are. Then search for “malpractice.” Now search for “lawyers” again. This time the ads will be for lawyers who specialize in malpractice.

So far this is largely harmless. It’s hard to imagine any violation that comes from Google having access to what you did 30 seconds before. What’s interesting is what comes next. As Google moves to place advertising on sites like MySpace, which have no natural advertisers, there is ever more pressure for it to use other sources of information to raise the prices at which it can sell those ads. Google is too quantitative — and it has too many engineers hanging around — not to be trying to calculate the extra money it would earn by using behavioral data for ad targeting. It also knows that it is a company in a fishbowl and anything it does that smacks of privacy invasion will cause a storm of comment and likely protest.

Most significantly, Google would be foolish to do anything that highlighted the way it could use its data until it receives final approval to buy DoubleClick.

One other thought here: Google is a lightning rod for debate about privacy because it is extending so quickly into so many areas. But there are so many other companies that are far nosier about what you do online and are unafraid to exploit that information. (I wrote about this last year, and activity in targeting has gotten more intense since then.)

From the start, Yahoo has seen itself as a company that uses data about users for the benefit of advertisers. And Yahoo already uses what you search for to pick which ads to show you on other parts of its site.

What’s more, there are advertising networks most people have never heard of (including Tacoda and Advertising.com, both owned by AOL, and BlueLithium, recently bought by Yahoo) that are in the business of collecting data about Internet users for advertising. Even creepier, Internet service providers are starting to monitor everything their users do to funnel ads to them.

All this is not to say that there is anything wrong with what Ask is doing. Some people may well want to search on a site that says it won’t remember anything about what they do. But the issues of what data is collected and how it is used is are far more relevant for Google, Yahoo, and a bunch of firms that are hidden from view.