Google + was a reaction to Facebook's success and its ability to keep Google off its data. If Facebook had not existed, Google + would not exist either, at least not before another big, entrenched incumbent.

Google created Orkut, an initially successful social network years before the rise of Facebook. Orkut died from scaling issues, something Google is really skilled at dealing with, had they really wanted to.

I agree with your main point that something else could have been Facebook if not for Facebook.