At the time Google+ was started and shortly after, leadership (larry page at that time) focused the attention of the company on it. There was a social bonus (that you'd get if you integrated your product), there were large changes to existing systems to support Google+, and the company made it quite clear it thought that social was the direction to go and that Google+ was going to be an enormous product.

I and a lot of other googlers were really confused by all of this because at the time we were advocating that Google put more effort into its nascent cloud business (often to get the reply "but we already have appengine" or "cloud isn't as profitable as ads") and that social, while getting a lot of attention, wasn't really a good business for google to be in (with a few exceptions like Orkut and Youtube, Google's attempts at social have been pretty uninspired).

There were even books written at the time that said Google looked lazy and slow and that Meta was going to eat their lunch. But shortly after Google+ tanked, Google really began to focus on Cloud (in a way that pissed off a lot of Googlers in the same way Google+ did- by taking resources and attention from other projects). Now, Meta looks like its going to have a challenging future while Google is on to achieving what Larry Page originally intended: a reliable revenue stream that is reinvested into development of true AI.

Google completely fumbled Google+ by doing a slow invite only launch.

The hype when it was first coming to market was intense. But then nobody could get access because they heavily restricted sign ups.

By the time it was in "open beta" (IIRC like 6-7 mos later), the hype had long died and nobody cared about it anymore.

They put a lot of effort into it, but it never had much usage.