Serious question: you build your own social network based on what people really want it too work like, and won’t track them or allow advertising, instead you invite just a handful of “promoters” every year to show same ad to everyone (to pay your bills) and build in tools that will accept facebook export format - how do you crank-start such network to beat an obvious chicken-and-egg problem?
Google doesn't need to crank-start anything. They just release a new product with invite codes and people go nuts trying to get them. If the product is good, people will use it. If it's not, they won't, in the end. G+ wasn't good.
They built a platform designed with no sharper focus than "for everyone", because that's what will supposedly generate the DAUs. But you have to try to bolt community on top of it and that's what generates the actual stickiness.
There are a galaxy of far stronger communities out there-- ones that were there long before Google+ and will be there long after. It's because they're clearly focused, themed communities-- newsgroups and forums.
If you provide some thread to bind the membership-- whether it's "People who Maintain Vintage Sansui Stereo Equipment", "People Who Really Liked The Animated Series 'Gargoyles'" or "Class of 1977 at Francisco Franco High School", it means people will generate content compelling to future users, and come onboard with attainable expectations.
These communities solved a lot of the problems that stymie many hypothetical Facebook killers:
* Selling new users. Just post the archives-- you can sell what you have, rather than promising "if we get aggressive enough and steer people into inviting enough loose acquaintances, somehow magic will happen and real community appears."
* Since they're more-or-less decentralized, you get very nice data partition models. On the outgoing side, you know that your messages are clearly restricted to one circle of potential viewers by default. On the incoming side, you're either viewing one community at a time, or aggregating yourself via RSS or the like under rules you know and understand. You're not going to be worried about a NSFW moment if you pop open "N-Scale Model Railway Enthusiasts Forum".
* The psychographic mix. You started with a clear message of value for participation, a give and take. In contrast, the classic social media feed appeals, by default, to a very specific type of self-promoter/narcissist type who just wants to bellow into the void "Look at me eating a burrito by the Eiffel Tower" and wait for validation. Think of all the people who never got on FB/Twitter/etc. because they saw no value in this type of interaction, or joined but have just a vacant profile after realizing it wasn't offering anything for them.
What the "community first" model doesn't do is scale to infinity. There are only so many people with any given interest. I think this is why Reddit is a success-- it's got a feel like a constellation of independent forums, that happen to use the same tech backend and SSO.