"... someone was able to find out my identity, which they proceeded to post. Do you know how it is possible for someone to find my identity from a Yahoo address?"
I don't know how to do it at Yahoo. I have assumed that it can be done, as also with anything associated with Microsoft Networks, AOL, Compuserve (which is now a subdivision of AOL) or any of the companies associated with the industry. This simply stands to reason ... it's intrinsic.
In each and every case, there is an administrative level at which these various levels of information are available. Simply as a consequence of human error alone, it is unreasonable to expect them to be successfully impermeable to penetration by people with unknown or incidental opportunistic agendas, who in most cases will continue to gravitate toward the highest position they can reach by whatever means necessary because that's what they're doing anyway.
For an employer, this is completely unsurmountable and usually requires miraculous intervention to even detect. For a careful administrator who notices a connection between information and something he's also interested in "should opportunity present itself," this presents no risk at all. This is simply in the nature of how we humans do things, and some people have additional help that we don't have, can't use, or spurn.
Knowing this, many people only write false information to the Web when they are setting up these Web-based email accounts and subscriptions and memberships. Investigators love that ~ it gives them "probable cause" to be suspicious, and can be used to get a search warrant or a subpoena. The last thing it is ~ is secure, or any privacy protection at all. It is also a factor in how your privacy is regarded within the industry.
This kind of thing occurs, remember, in an environment where "industrial espionage" is standard business practice. "Preventing It" is the name of the college course, doing it is the course content. It pays well. An occasional commotion about a privacy protection failure is not something that will threaten the business very much. In guarding against industrial espionage, this would have a very low priority. The risk in this particular breach of trust is nearly nonexistent, and the mere presence of known-to-be-bogus information in the database makes the risk even less. What exists on the Web labelled "privacy information" is known to be half-false, it gains no real respect.
After these considerations we turn to cracking as a possibility, someone gaining illegal access from outside the company, as Kevin and others have demonstrated can be done, and is the kind of thing that continues to appear regularly all over the world as prosecutions and sentencings ~ go directly to jail and do not pass GO stuff. Actually, though, very little of this actually is even detected ~ people acquire the information they want for their own purposes and it never goes any farther ~ like getting posted on the Web somewhere. The serious kind of illegal data acquisition is done by professionals who don't get caught. It's merely another kind of industrial espionage and goes on all the time. And again, it pays well.
The industry, of course, will regularly say that it cannot be done. This is patently false on its face, whoever of them says that is quite knowingly lying. Trying to stop it ~ and failing regularly ~ is a multi-billion-dollar industry all by itself now. Then they will say that it is nearly impossible and they're working on it. This also is false, but it is possible that the speaker doesn't realize that, there is calculated and deliberate ignorance in such positions (of being spokesmen for the industry or a company). Showing how it is known within the industry conclusively to be false is beyond what I wish to do here tonight. Then, when confronted with irrefutable evidence that it has been done, or more specifically some public clamor about such a confrontation, they will announce that the hole has been closed and the perpetrator apprehended (or about to be) and prosecuted, and that clients don't need to worry about it any more. And buy some silence. That can pay well too, but not very often.
You see where this goes in an industry whose economic viability and potential rests on consumer confidence. Where it's necessary, liars are hired for that particular skill (lying). That's basically what got Bill Clinton elected ~ he was an entertaining and extremely skillful liar, almost to the level of art reached by Benjamini Netenyahu. At higher levels this is called diplomacy. In business it's called customer relations. "Building Confidence" is the course title, avoiding questions is the course content.
This is not cynicism, it is merely observation. Anyone can take two quick looks around and see it plainly unless they simply don't want to see it at all. We live in a world of fiction, lies and deceptions, and lie to ourselves, first, about that.
Your email address can be a search term and will bring up many of the pages from wherever you've left it. Your IP address, or (in the case of many consumer connections) an IP range, can be searched at Google and elsewhere. Anything you sent anywhere that left an IP address that can be found ~ eMail headers, posts to newsgroups, etc ~ can be followed. Through that kind of searching, and there are extensive databases of considerable variety in which to search, just about anyone can be identified and located. It can be a very tedious enterprise or it can be trivial, depending on how the search goes.
These methods do not breach any trust at all, are not unethical, and are all perfectly legal. They are commonplace, and within the capacity of anyone with a Windows machine (which contains a sufficient toolkit) or any other machine, by anyone who gathers the tools and learns how to use them. And there are websites that have the tools available to use on-line, it's not even necessary to know how to use the tool as it exists in your DOS directory.
This should be the first investigation for you, because you are the person who knows where you have gone on the Web and how ~ you know where to look to see what traces you may have left. Go try to find yourself on the Web ~ that is, if you already know who you are. It's very informative.