By which metrics was Google+ successful? I don’t doubt you, I just never heard that before.

I can't say user metrics but Google Photos came out of Google+ and it is a massively successful product.

Hangouts (now Meet?) was also a product out of Google+ but considering GChat (the xmpp version) existed before, it was kind of a step backwards.

I think Google Local Guide also has some parts of Google+, not sure.