Another mistake is that they had a significant presence in Brazil through Orkut, but they didn’t bother to integrate and migrate the users in.
Orkut’s user base was already degraded through Facebook but it was not inexistent, as some features of Orkut were unique. One was that it allowed people to use alt accounts to participate on anonymous discussion, not much different from Reddit, I’m sure with some creativeness G+ could have benefited from extra users.
Orkut Büyükkökten's orkut.com might have been a semi-private project (Google, like any company, doesn't normally name its products after first names of its coders).
Also, as you say, it was populated by many Brazilians, an imbalance like that may discourage non-Brazilians from joining, so not sure if integrating them would have helped or hurt.
Orkut’s user base was already degraded through Facebook but it was not inexistent, as some features of Orkut were unique. One was that it allowed people to use alt accounts to participate on anonymous discussion, not much different from Reddit, I’m sure with some creativeness G+ could have benefited from extra users.