I have to concur that the silence (in a variety of online participatory spaces I’m involved in) is troubling.
Being something of a pessimist, I generally interpret the silence as absence. Occasional emails about the myriad of bizarre life circumstances that prevent people from participating in something that they had planned to would seem to reinforce this.
But then, every once in a while, I get a passion-filled comment from someone who I thought was MIA — expressing deep learning, value, appreciation, etc. This leaves me even more perplexed.