G+ feels more like going to some sort of gathering where everybody is off in groups talking quietly amongst themselves. It's not empty; it's just not very interesting if you don't know anybody there.

For example, the developers and researchers of compilers for functional programming languages are there and active, but scrolling through the posts, I've noticed that all of us (myself included) tend only to make the posts visible to other compiler developers.

Trying to think about why that is, I can only assume it's because of the background knowledge problem. If I want to talk about random things I'm doing in our compiler to this group, I can just type up a thousand words. If I wanted to do it publicly and have it understandable, I'd have to make a Matt Might-style blog post, and the only way I would do that is if, like him, I was already preparing the contents for undergraduate consumption. Or maybe if it was content I had prepared for a paper that was too introductory and had to be cut due to page restrictions.