It's an important question because it helps us understand where Google and G+ is going.
Facebook has commandeered people's time online. They're spending all their time on FB, and therefore creating content and filtering it through their own eyes.
You mentioned FB was worried about being commoditized. Maybe. I think FB thinks that the analysis of FB interactions with products/contents/brands/friends is its treasure and didn't want Google to come in and out-algorithmize (not a word i know) them for analysis. Their thinking: "our" content should not be used by Google to personalize advertising on Google.com.
However, I think FB has one thing fundamentally wrong. It believes that interactions and content from its users on FB is theirs, not the users'
. Our photos are not theirs, they're ours. By thinking that way, it goes against the modo of the scattered internet. This I believe will lead to their downfall.
Why? I'm personally seeing trends that social is de-centralizing, it seems to be going distributed & specialized w/ services like pinterest, path, tumblr and even reddit, chime-in, quora, etc... FB or G+ cannot hope to have all our social answers. They can no longer hope to be the one stop shop. For now, pinterest and others have tagged along with facebook logins and API plug-ins, but i don't believe that's a long term strategy because it makes those services submissive to Zuck & Co.
I think Google would have preferred to be a social aggregator, than than have its own service to collect the information it needs for tomorrow's search. But alas, FB did not open and forced Google's hand, and G+ as a result is improving every aspect of Google's services. Thanks Facebook!