In order for people to actively use a social network such as this, they need a reason. That reason (keyword "social") is /friends/. If you don't have any friends using it, why would you? If you aren't using it, why would they? This creates a very difficult transition that only really works with a mass exodus, similar to what happened to MySpace.
MySpace was still enormous when Facebook *really* started going mainstream. Maybe a year after Facebook saw it's popularity skyrocket, MySpace became yesterday's news. That is a mass exodus.
What caused it? I believe it was simply the interface. Myspace was stupidly annoying. Going to people's pages and being bombarded with glitter letters, obnoxious colors and backgrounds, blaring music...it just wasn't fun. They provided *way* too much customization. You also had to visit people's pages in order to really keep up with them.
Enter Facebook. News feeds - you could see what all of your friends said in one neat, well formatted page. You almost never had to visit somebody else's profile, and if you did, it was easy to read. Facebook twitterized MySpace and threw in their own spin on things. This was a big enough difference, so when it came time - people left in droves. I for one, knew about the site for a while. As soon as a good number of my friends moved, I moved. When I moved, shortly thereafter I saw other friends move. That's how it works.
Google+ is awesome. I love it far more than FB. Unfortunately, for the average user, it just isn't "different" enough to cause that kind of mass exodus. Maybe sometime in the future Google will do something to push it over the ledge and take over the market - but until then, Facebook is king of the jungle.