Setting plans to meet, but never showing up nor apologizing.
Dates announcing an intention to meet, then ignoring any follow-up about plans.
Dates ignoring e-mails sent them or even responses to their initial e-mails.
Much of this behavior is rude, and there's little explanation other than lack of decency. After all, it's much easier to cordially say no by e-mail than to do it by phone call.

So here are some rules?developed after discussion with other Internet daters?regarding what situations deserve a response.

After your first contact.
If someone contacts you, you don't have to respond. Some dating sites do ask you to send a ?decline contact? auto-reply. I think a response?whether simply an auto-reply or a brief note'is appropriate if the approach was sincere and the person reasonably in your ballpark. However, if the person who contacted you sent a form letter, or was clearly outside your stated parameters (age, geography, etc.), then a reply isn't necessary.

It's always good to stress the unpredictability of chemistry, rather than your analysis of their faults. So write ?Thanks for your note, but I don't think we're a match,? rather than, ?Thanks for your note, but you're way too fat/bald/poor/materialistic for me.?

If you change your mind before your date.
It's possible that you might have exchanged an e-mail or two?even a call?but have changed your mind. Maybe you've learned some more about them and realize the potential has diminished. Or maybe you've met a better date.

Here you obviously have to be more careful, because earlier in the sequence you did entertain the possibility that a match was possible. Try something like ?I'm sorry, but I now think we're not as good a match as I hoped, so I'll wish you luck.?

After a meeting.
It's OK, after one date or even two, to simply let things lapse. Mutual silence speaks for itself. However, if one of you does follow up, it's rude to just ignore it. Better to simply say something like, ?I'm glad we made the effort to meet, but I don't think we're a match.?

What if someone violates these rules? Yes, it's tempting to upbraid them. But they've already moved on, so why waste your time on someone who clearly isn't right for you? Try to focus on a new person, and assume the best, until proven otherwise.

Bad Internet manners feed on themselves. Once you've been treated rudely, you might think it's OK to treat the next person badly. Don't let that happen. A little courtesy goes a long way. And karma is a boomerang.