Social media is all about conversation. Or, rather, conversations, in plural. If it was just the one conversation, we could apply the old standard conventions to it and go from there. But it isn't, and we can't.
Instead, there are several emerging new conventions, most of them emerging in parallel and with some overlaps and areas for conflicting expectations.
One thing everyone seems to be on board with is that there are, in fact, several conversations going on at the same time. Not one, not two, but a whole bunch of them. No conflicts here. Until it's time to end one of these conversations, for whatever reason.
Some people (me included) are of the opinion that you can't reasonably be expected to talk to everyone all the time, and that conversations naturally fade away, only to be reopened again when new topics arrive. It's not so much one single conversational thread going on from start to finish, as it is a continuous string of interaction about whatever comes along. Sometimes, exchanges happen; sometimes, they don't. But whenever something pops up, the social space includes the option of saying "hey, look at this, what do you think?".
The subtle shift from "conversation" to "social relation" inherent in this view are not coincidental. It is the very essence of the social media thing.
Not responding to a particular input is not a big deal in this view. There will be more inputs, and more things will be said. The social relation will endure when any one particular conversation ends.
As you might imagine, there are other opinions, other conventions. Some of which are much stricter on when you can legitimately not respond to things, and which social markers you need to employ to disengage from a conversation in a proper manner. In fact, they closely resemble the old style rules of conversation: you are talking to someone, with someone, and this deserves the respect the situation warrants. No matter the number of conversations.
The analogy here would be to hang up on someone mid-sentence - that simply will not do.
I've already proclaimed myself partial to the first way of looking at things. This doesn't mean it's the right, only and true way - it's just useful information for those who want to talk at me (@sargoth, hi, talk at me). I'm just the one person, and by no means in a majority. But you will run in to people from my neck of the woods, and you will run in to people from the traditional woods of the neck, and you will most likely have social media conversations with both of them at the same time. Knowing that there is a difference will help you avoid those subtle social transgressions that so often happen when different conventions meet up and talk to each other.
Social media is all about conversation. And conversations. And conventions. And [the blog post fades to a close, opening up a social space for commenting and conversing]