Your reasoning about the potential threat of Facebook is of course totally cogent, and indeed Facebook could be a threat to Google's business. The only problem is that they haven't proven there's a way to monetize that word-of-mouth that Facebook technologizes so well (because of course it's always existed). In fact there's a strong argument to be made that it is impossible to monetize this phenomenon without destroying it. Facebook is throwing everything they have at this problem, but they have still not crossed the chasm. The state of play today is that AdWords are expensive because of the ROI they can drive, and Facebook tend to be cheap, yet still overpriced due to a wave of hype that has crested but still hasn't washed away back to sanity yet. And when the smoke clears will Facebook's model be more valuable than Google's? I think that's anything but a foregone conclusion.

Given the uncertainty of the winning model, I'd rather see Google stay true to their DNA rather than chase after the Facebook hype with the G+ strategy. Some good has come out of it in terms of improving their authentication across properties, but by and large it appears as an impotent move to recreate a second-class implementation of Facebook that no one gives a shit about. Meanwhile, Google actually has tons of properties that Facebook can't touch. So why are they chasing after an upstart? I consider it a sign of weakness and ultimately detrimental to try to shift your company culture like this. If you're on top with a certain strategy, do your best to ride that wave instead of running scared and trying to compete with someone else on their terms. It's a sign of weakness, and only should be pursued when the company is in real trouble, not based on attempted clairvoyance.