I don't think that is true - success of Skyrim (or other TES series) stand in direct contrast. FPS are indeed non-moddable nowdays but FPS have been done to death and only the franchises (like Modern Warfare, CoD, Crysis) are making enough money.
As per Gibson: "The street find's it own uses for things," and it's that ability of the street to do so that gives long renewing interest into the thing. Either let oneself be reborn again and again, or die of certain irrelevancy.
At some point you have to consider the fact that there really isn't much left to do in FPS. When Quake and HL came we were still in infancy. While CS survived every other mod died a horrible death in neglect. People now know what sucks. And what doesn't has been honed to perfection and there really isn't much left to do anymore.
If you want to see modding don't look at FPS, look at Minecraft.
There is a Moments API for external applications to post specialized content into the feed, and there is a read-api giving an Activity Streams centric feed.
Conversely, perhaps it's that we're less likely as a civilization to try to push things to interesting uses. Quake1, Quake2 and Unreal Tournament mark an exemplary period where dabblers came along and tried to push the systems, tried to find interesting and creative twists. Twitter was similarly buoyed into popularity, but today we see polished, final games- games are no longer moddable, would-be hackers simply license and leverae the engine, not a complete game to hack from, and like we see movements and projects such as IndieWeb, Tent.io, app.net, Salmon, ad-infinitum saying we must do it ourselves, rather than focusing on creating a rich dynamic around something that is.
Suffice it to say, my view is that anything that attempts to stand alone is bound by very low limits, and I'm not sure where G+ needs to go to overcome it's limits of being merely itself.