Zuck should have read Sun Tzu instead of Cato - "If you wait by the river long enough, the bodies of your enemies will float by." I'm not sure how much of the G+ failure can really be attributed to any particular activity of FB as a company.
The "social must be everything" Mantra that Google was chanting then was annoying and ridiculous. But part of the situation was the company didn't want to just create a useful service for people but rather wanted to impose a situation where people's identities could be monetized by Google.
Facebook has had trouble monetizing their product and I suspect they're safe because every company with the resources to produce a UI and service as useful as Facebook is going to want have such a service impose more conditions, be oriented towards quicker monetization, etc.
Even MySpace could have persisted if they'd focused on providing things Facebook didn't - but no one wanted to extend credit to them to do and creating this stuff apparently costs a lot of money for a significant amount of time.
But G+ seemed to push the strain quite high.