Think of the Japanese kids sending empty SMSs to each other, and this is a natural progression of technology. It has to be quite explicit to begin with, that we're just doing a "quick hello" but as we understand the affordances of the medium usage changes.
There's a form of SMS now which doesn't solicit any kind of reply, just a "hello, this is what I've been doing today" -- this is fairly new.
There used to be a form of email that ended with "NRN" (no response necessary) -- but this itself isn't necessary now because we understand emails don't need to be replied to. Or rather, we can always pretend we weren't able to respond.
There's this thing with IM: go online, send a link, disappear again. As we understand it better we'll develop a social framework that says what sort of IMs are replied to and what aren't.
If we're twisting existing technologies for this semantic-free but meaning-rich communication, if we're finding that as humans we require it present, shouldn't we build software to support that use explicitly?