I use the level of "is the response worth someone stopping walking down the street, taking their phone out, unlocking it and reading it" (this is the same line I use for text messages, if it's not worth someone's while to do such a small task as stopping and reading it then it's more than likely not going to be missed if you don't post it).

I'm sure some of my posts fall short of that from time to time but it's at least something I try and think about.