There are, of course, many World Wide Web based personals services. All I will say is buyer beware. Alternatively, you might want to put up a personal homepage. You can say as much or as little as you want and it won't expire like most personal ads. In some cases, you can remain reasonably anonymous, if that's what you wish.
Most service providers offer web spaces at no extra charge with Internet accounts.
This really just applies to Usenet, not the Web. It is a feature in some news reading software to filter out unwanted messages. You tell it not to show you articles containing certain words or phrases or posted by certain people. It can be helpful in some places, but until more people start using simple, easily identifiable titles to their personal ads, this will remain a mostly-useless feature for personal ad readers. This feature is most common to Usenet, but you do see it in some of the Web services.