You hit on possibly the defining difference between G+, Facebook and others.
That is that it allowed you to make new contacts and along lines of common interest. The Communities aspect was, for me at least, such a rewarding way to expand my social horizons, ask questions, learn new things.
As I have seen said elsewhere, Facebook is really rooted in a "friends and family" perspective and, whilst very nice on its way, it does not really grow one's social footprint.
Quite why G+ had this edge is probably down to a bunch of things but for me the UI is a large factors. The way the G+ functionality worked encouraged a outward looking experience. I am sure I am not the only one to find the Facebook UI rather ugly, cumbersome and unpleasant to use.