I didn't just keep posting to Twitter and Facebook for nothing. There was just enough feedback to keep me posting continually. The key to Pavlovian training is the eventual randomness and pace of the rewards. The pigeon will keep tapping that button until the pellet comes out, even when it happens more and more slowly - in fact the pigeon will increase its effort as the pellets slow. Same thing with status updates?
...Is everyone posting to Twitter and Facebook simply because everyone is posting to Twitter and Facebook?
If you're commodity content to your friends don't be surprised when they don't notice when you disappear.
The seldom-mentioned corollary of having a large number of "friends" is that those same "friends" of yours probably also have a large number of "friends" who will drown away your absence. It's easier to have ten of ten total friends notice when you're gone (100%) than to find ten out of a million (.001%) who actually care.