My poll was even less scientific as usual, for I forgot to leave room for control groups. Alas. Well, anyway, 40 people (not a big slice of my daily readership) responded to the "Online Romance Suddenly Over Without Explanation" poll, 36 of them women and 4 of them men. Of the women, 28 have suddenly discovered internet silence where a man used to be, and 8 have done disappearing acts themselves. Of the 4 men, 3 have been abandoned, and one did the abandoning.
For readers' take on internet dating, see most of the comments here.
I am not sure what to say other than that unless you are actually frightened of a person, it is very disrespectful behaviour to abandon a friendly relationship--even an online friendly relationship--without an explanation. "I'm just not feeling a spark" counts as an explanation. "I'm not comfortable with your anger" does, too, if that's the problem.
If a very embarrassing situation has cropped up, like you have discovered that Mr Perfect was your little sister's hapless prom date, well, this is the sort of thing that separates the women from the flibbertigibbets. You should explain the situation, being straight to the point. Men tell me that they'd rather be told the truth then left hanging. So tell them the truth.
But don't tell them everything about yourself online. A lot of women have the bad habit of telling strangers our business, and online it strikes me as the equivalent of telling a man the end of a thriller just while he is absorbed in Chapter 2. If he really wants to get to know you, and if you really want to get to know him, you can darn well meet down at the doughnut shop. If you live in South Bend, and he lives in Boston, well, you're going to have to compromise on which doughnut shop.
There was a comment that worried about leading a man on by accepting three dates with him. I don't think that is leading a man on. Making a man think you might go out with him when you know you won't is leading a man on. Making him think you might sleep with him when you know you won't is leading him on. Making him think you might marry him when you know you won't is leading him on. Everything else is just you saying "Yes" to stuff you actually want to do. As long as whatever it is (e.g. going to a film) is morally licit, there's no problem.
One of the odder things about women, I have noticed, is that we tend to feel guilty about stuff we shouldn't feel guilty about and then not guilty about stuff for which we should feel guilty. If a man flies to your city to meet you, and then you don't fall in love with him, you shouldn't feel guilty about that. If a man flies to your city to meet you, takes you to dinner, you are smitten, he regretfully confesses he isn't smitten back, and in your hurt you tell everyone he led you on, you should feel guilty for that.