I feel you on this.
Online, without a real life connection, its easy to let things slide. I totally agree that it feels like lying if someone says they'll do something and they don't, but they probably aren't doing it to deliberately hurt you, even though it really really hurts - they probably have no idea. If someone tells me they'll call me and they don't, I'll usually hold the phone waiting for their call, or get so excited about their call that when it doesn't happen, its totally devastating. But they're not thinking anything about it on their end - they don't imagine anyone is sitting there waiting for a call or reply or anything else b/c to a NT it seems so small.
I've tried to be upfront with people who do this to me, and tell them not to promise to do things, to be deliberately vague, to always make sure to use qualifiers like "maybe" or "I'll try", just so I'm not waiting for them to fulfill their word and so I don't get disproportionately upset.
If you're pushing people to comment or make promises to you, then they might just be saying yes to be polite, but really have no interest. This is harder to control. because everyone means well, just be careful to make sure you're not pushing and things should get better.
If they're doing it out of the blue - well, people used to call me and invite me somewhere, then forget and be confused when I confronted them about not being there. Just remember - they are in the wrong, but there is usually no way to make them understand the gravity of what they did. If someone does this to you more than once and doesn't really apologize, disregard them. Don't value them with your trust. Be distant until they decide they really want you as a friend and behave in a way that values you. AND DONT LET THEM CONVINCE YOU IT IS YOUR FAULT FOR HOW THEY TREAT YOU. It isn't your fault. You are you and are only responsible for what you do and say, you are responsible for no one else. If you take the power back and let them see you wont let yourself be mistreated (and try not to attack them or call them names, because this gives them the power back), things usually go somewhat better socially.
One of my 'obsessions' or 'areas of expertise' is movies/film criticism/film history, so tonight as Oscar night is literally one of the biggest highlights of my year, and I love trying to connect with other knowledgeable people online during the broadcast via social networking sites like Twitter. But what sucks about being obsessed with film is that it isn't peculiar enough to stand out, yet everyone else thinks they're movie experts, too. Unless they made notebooks in 8th grade with cast lists and quote lists and interviews and articles that they spent hours and hours studying, I doubt they're coming from the same place.
Its completely hard, and a bit of me thinks "who am I to say this?" because at 26 I just burst into tears because one acquaintance of a sort-of-friend twittered loudly how awful and boring my posts were and someone I liked just fine unfollowed me on Twitter for posting too much during the Awards. I had been so excited, and when I get excited I forget what social boundaries I've learned and just let it flow, then people I could care less about in the real world but am linked to online go out of their way to deliberately ruin my night or detach from me forever.
Its easy to snub or ignore people when you're only acquainted online. That doesn't make it ok at all, but it makes it easier. I'm sure many many more people were thinking it about me tonight, but I'd hope they're familiar enough with me to know that Oscar night is special enough to me to let it slide.
I know it sounds counter-productive to those of us who really want and need friendship connections, but having no friends, in the long run, is better than having loads of half-friends who abuse, don't understand and undervalue you. Not that you should rid yourselves of the people you're asking about, but if you are straightforward with them about how breaking promises feels and they don't stop, then let them fall into the bad category.
Or at least that is what my own really sucky experiences have taught me. People can, do, and will always to some degree suck.